There's no reason not to squeeze.....that's another old brewer's myth that has been misunderstood...and has been shot down..But if often just get's repeated as ROTE without anyone stopping to look beyond the just repeating the warning...
Read this
https://www.homebrewtalk.com/f36/do-you-squeeze-bag-biab-177051/?highlight=squeeze
And this.
https://www.homebrewtalk.com/f39/squeezing-grain-bag-bad-175179/?highlight=squeeze
From Aussie Homebrewer.com
Tannins And Astringency
If you are worried about squeezing your bag too much or crushing too fine, relax! Astringent beers do not come from finely crushed or squeezed husks but come rather from a combination of high temperatures and high pH. These conditions pull the polyhenols out of the husk. The higher your pH and the higher temperature you expose your grain to, the worse the problem becomes. Any brewer, traditional or BIAB, should never let these conditions arrive. If you do allow these conditions to arrive, then you will find yourself in exactly the same position as a traditional brewer. Many commercial breweries actually hammer mill their grain to powder for use in mash filter systems because they have control of their pH and temperatures. This control (and obviously expensive complex equipment) allows them non-astringent beers and “into kettle,” efficiencies of over 100%.
As long as you keep your steeping temps below 170, you won't be producing those supposed tannins that folks blindly say you would be squeezing out.
1) If your PH is off, or your steeping/mashing temp is above 170, your beer will extract tanins from the husks whether you squeeze or not
2) If your PH is ok, and your temps were below 170, squeeze away!
There's been some tests that have disproved the whole "don't squeeze the grain bag, because you will leech tannins" idea. I think there's even been a couple experiments on here detailed in threads. I think it's been pretty well shot down as one of those "old school" beliefs, that turn out to have little effect.
In fact if you are doing AG "Brew in a Bag" you are encouraged to squeeze the grain bag. They even showed it on basic brewing recently, the took a ladder with a hook attached, hung the grain bag, and twisted the hell out of it to drain every ounce of precious wort out of bag of grain.
This should launch as an mp-4
http://media.libsyn.com/media/basicbrewing/bbv01-16-10cornpils.mp4
So is that's the case, that it is "OK" to do in AG Brew in the bag, then why would it really be bad in extract with grains brewing?
I wouldn't worry about it.
From BYO, MR Wizard;
The two most influential factors affecting the extraction of tannins from malt into wort are pH and temperature. All-grain brewers are very careful not to allow wort pH to reach more than about pH 6 during sparging because tannin extraction increases with pH. In all-grain brewing wort pH typically rises during the last stages of wort collection and is one of the factors letting the brewer know that wort collection should be stopped.....
Temperature also affects tannin extraction. This relationship is pretty simple. If you don’t want to run the risk of getting too much tannin in your wort, keep the temperature just below 170° F.
This is where the answer to your last question begins. You ask whether steeping and sparging released "unwanted tannins" in your beer. For starters, all beer contains tannins. Some tannins are implicated in haze and some lend astringent flavors to beer.
The type most homebrewers are concerned about are those affecting flavor. In any case, it is up to the brewer to decide if the level of tannins in their beer is too high. The (in)famous decoction mash is frequently recommended when a brewer is in search of more malt flavor. Decoction mashes boil malt and — among analytical brewers who are not afraid of rocking the boat with unpopular ideas — are known to increase the astringent character associated with tannins. In general I wouldn’t consider 170° F dangerously high with respect to tannin extraction. However, if you believe your beers may suffer because of too much astringency, consider adjusting your steep pH and lowering the temperature a few degrees.
They are often repeated ad nauseum by, especially new brewers, with little know understanding of the context behind them...or even a basic thinking like, "how come it says not to boil your grains, yet people doing decotion mashing do it all the time?" or "They say not to squeeze their grain bag, but in Brew in a Bag- they are encouraged to squeeze them...so what's going on here?."
It's the same with boiling your grains... posted a detailed discussion of the "chestnut" here;
https://www.homebrewtalk.com/f39/less-than-satisfied-my-first-brew-224679/#post2639410
(There are actually a number of instances where what's been told about that is actually done in all grain brewing....)
In fact I'm boiling my grains right here (It's called decocting)
