From what I understand, steeping too long will extract astringency. This is due to the husk material. You won't get any additional flavor. The same principle holds true for squeezing the grain bag. Just steep, then rinse, then boil. For AG brewers, over-steeping the specialty grains would be the same as over-sparging. After so much, you've gotten all you're gonna get out of the grains. After that, you start getting stuff you don't want. Just what I've heard.