1) It's really a good idea to make starters when using ANY liguid yeast for all beers above 1.020 OG...
The biggest reason I suggest folks make a starter is if you make one you'll have peace of mind.
And you won't be starting an "is my yeast dead" thread in a couple of days. (Which then one of us will answer with...."Had you made a starter..."
Making a starter first insures that your yeast is still alive and viable before you dump it in your beer. You will be less likely to start one of those "is my yeast dead?" threads that are on here every day.
You will also ensure that you have enough yeast usually the tubes and smack packs are a lot less yeast that you really should use for healthy fermentation.
Making a starter also usually means your beer will take off sooner, because the first thing that the little buggers do in the presence of wort (whether in a flask or in a fermenter) is have an orgy to reproduce enough cells to do the job...So it won't take such a long time in the fermenter since they started doing it in the flask.
Additionally it is better for the yeast to consume and reproduce incrementally rather than just dumping them into the fermenter...The yeast will be less stressed out than if you just dump them in.
Stressed out yeast can lead to a lot of off flavors...maybe even (though rare) the dreaded autolysis....Or the curse of 1.030....getting a stuck fermentation because the yeast have bit the dust.
So making a starter proves your yeast is still healthy, allows you to grow enough yeast to do the job, cuts down on lag time, and ensures that you will not get off flavors or stuck ferementations from stressed out yeast.
Also has to do with the actual pitch rates of the smack packs and tubes, and has to do with the data that Jamil Z has on his mr malty website.
I'll quote some of it, but really you should look at the stuff there;
Ales & Lagers
The general consensus on pitching rates is that you want to pitch around 1 million cells of viable yeast, for every milliliter of wort, for every degree plato. A little less for an ale, a little more for a lager. George Fix states about 1.5 million for a lager and 0.75 million for an ale in his book, An Analysis of Brewing Techniques. Other literature cites a slightly higher amount. I'm going with Fix's numbers and that is what the pitching calculator uses.
If you're curious, here is the simple math to calculate the number of cells needed. For an ale, you want to pitch around 0.75 million cells of viable yeast (0.75 million for an ale, 1.5 million for a lager), for every milliliter of wort, for every degree plato.
(0.75 million) X (milliliters of wort) X (degrees Plato of the wort)
* There is about 3785 milliliters in a gallon. There are about 20,000 milliliters in 5.25 gallons.
* A degree Plato is about 1.004 of original gravity. Just divide the OG by 4 to get Plato (e.g., 1.048 is 12 degrees Plato).
So, for a 1.048 wort pitching into 5.25 gallons you need about 180 billion cells.
(750,000) X (20,000) X (12) = 180,000,000,000
As an easy to remember rough estimate, you need about 15 billion cells for each degree Plato or about 4 billion cells for each point of OG when pitching into a little over 5 gallons of wort. If you want a quick way of doing a back of the envelope estimate, that is really close to 0.75 billion cells for each point of gravity per gallon of wort. Double that to 1.5 billion for a lager.
Pitching From Tubes, Packs, or Dry Yeast
Both White Labs and Wyeast make fantastic products and you can't go wrong with either one. There are differences between their strains and each brand has pluses and minuses yet neither is better than the other across the board. Use the brand your local homebrew shop carries, if you need a way to decide.
A White Labs tube has between 70 and 120 billion cells of 100% viable yeast, depending on the yeast strain. Some cells are much larger than others and there are more or less per ml based on size. (The information on the White Labs web site stating 30 to 50 billion cells is out of date.) We can just assume there are around 100 billion very healthy yeast. You would need 2 tubes if you were pitching directly into 5.5 gallons of 1.048 wort to get the proper cell counts.
A Wyeast Activator pack (the really big ones) and the pitchable tubes have an average of 100 billion cells of 100% viable yeast. The smaller packs are around 15-18 billion cells. You would need 2 of the large packs if you were pitching directly into 5.5 gallons of 1.048 wort to get the proper cell counts. For the small packs, you'd need eleven of them!
But to make it easier he has a great pitch rate calculator http://www.mrmalty.com/calc/calc.html
And according to his numbers on his calculator, really any beer above 1.020, you should be making a starter for.
Me personally when I use liquid yeast I just make a starter. I may not be as anal as some brewers and makes sure that I have the exact cellcount for whatever gravity beer I am making, but I do make one for the above reasons I mentioned, namely peace of mid, and a reduction in lag time.
2) About 95% of the stuck fermentation threads are really
"my airlock stopped bubbling, but I am too scared to take a hydro-reading to confirm what's going on so I am going to start a panic thread instead." And almost all of those, when they realize that an airlock is a vent to release excess co2 and not a magic fermentation gauge, and that a hydrometer is NOT something to fear, but your main diagnosis tool. And they take a gravity reading they find out their beer is perfectly fine.
The rest of them tend to be the dreaded 1.020 curse where the amount of fermentables left if very negligable and there is little left for the yeast to eat so the yeast stop working. The very few, and really rare ones will truly be a yeast pooping out in a large gravity beer.
To help insure that you don't have a stuck one.
1)Always make sure you use the right amount of yeast or even over pitch. That is WHY for liquid yeast you should make a starter. To make sure you have plenty of buggers to do the job.
2)Oxygen/aeration. Make sure you give the yeast plenty of oxygen before you pitch. You've boiled out the o2, so you NEED to re-introduce it. There are many ways, shaking the heck out of it, using an O2 bottle and stone, Aquarium pump, stirring it, and a bunch of other things you can look up. Just do something!
3) Yeast energizers and yeast nutrients are a good idea as well. You can buy various types of both from lhbs and online, and simple "yeast energizer" is to add some dead yeast during the boil. Yeast are cannibals, and they will eat their own first. So if you take any yeast including some bread yeast, which is cheap
and add about a teaspoon of it to the boil at some point (I usually do it in the last 15 minutes) that will kill it, and give the beer yeast something to snack on.
4) Simple sugars. Many recipes, especially big beers have a sugar addition or many throughout the boil and fermentation process. These less complex sugars get the yeast up and running. If they get excited they eat more.
If you do get a stuck fermentation, sometimes it takes a simple shake to gently lift the yeast up into suspension and get them working again.
Also raising the fermenter temps, even simply by tossing a blanket around the fermenter will get the yeast back up and running.
Just remember that MOST of the stuck fermentations have nothing to truly do with stuck fermentations, but folks thinking their airlock is more representative of fermentation than it really is. It is just a valve, and most of the time, the valve is NOT going to need to release excess co2, especially later on in the fermentation process.
Fermentation will wind down, bubbles will wind down (if they even start) but that doesn't mean anything is wrong.
Hope this helps