I always, always prewash every piece of fabric I put into a quilt because of potential differential shrinkage of fabrics not off the same bolt (big trouble if this happens) and because of big surprises I've gotten sometimes expecting some colors not to run and they did, horribly. I've eventually thrown away some blue and some purple charm squares because they just never stop bleeding and eventually it's not worth it to keep trying.
Also, I do most of my quilting for charities involving kids, and I don't want to make a quilt for a kid that might shrink out of shape or the colors might bleed and make it ugly. I want to know when it leaves my hands and goes to some unknown kid that he/she will have something to keep that will continue to be nice and comforting and pretty, not something that I didn't prewash the fabrics for because I liked the look of new fabrics better. Just my personal take on it!
Dana