According to Jim Crute at Lightning Brewery who does almost exclusively decoction, it is only certain grains which may be prone to releasing astringency (ones he does not typically use).
On the grain bag squeezing thing, I think its a big myth. If you look at process scale mash filters that some of the big (very big) brewers use, they end up compacting the mash into a highly compressed filter cake and then force water through it. I asked Charlie Papazian about that at a recent event and agreed that is not true. He pointed out the Goat Scrotum ale recipe in the Joy of Homebrewing book that specifies squeezing the bag as a counter argument.