A couple of good places to look for how much sugar to prime with, both for the brew style you have to bottle, and to the CO2 volumes are:
Beer Recipator
Bottle Priming Widget
I also use the aspect in Beer Smith, but check against at least one other site to see what I get. As long as the calculations are either the same, or close enough (I often split the difference, or make a judgement call depending on how big the difference is) then it doesn't matter.
You can get a good digital scale these days pretty damned cheap. So, there's no real reason to not weigh the sugar. The way I see it, if you've taken the time, effort, and care with the brew from the start, why not show that same amount of care at the finish? Also, with using one of the calculation tools, you really don't need to worry about over carbonating a batch. Well, unless you prime for 5 gallons and have less than 4 gallons of beer to bottle, and you make the priming solution for a high CO2 volume amount. Just a rough example... If you figure on getting 2.5 CO2 volumes, in 5 gallons of 66F brew, using 4-3/8oz of sugar. But, you only have 4 gallons of brew to actually prime and bottle (using the same amount of sugar), you'll actually get more like 2.93 CO2 volumes in the brew. Depending on the bottles you're using, that could be dangerously close to their failure point (or over their tolerances)...
I know there are plenty of people that have gotten away with just scooping sugar into a pot of water and priming with it, without bombs. Personally, I'd rather not risk it. I also want to make sure I'm priming the brew to where I want it to be. That extra amount of customization is just another aspect (IMO) of home brewing. Just like how people that keg set their pressures to where they want the brew to be (for carbonation, and serving)...