I never prewash, because:
1)I'm too lazy and don't like to iron before I did anything with the material.2) As it is, I have a hard time with cutting and sewing straight - I figure the stiff material helps control that. 3)I have too much material that isn't washed already, so what's the point.
HOWEVER, I made a quilt with dark blue, dark wine, and very, very white. Guess what happened? After I washed it, there was pink on the back. I might change my mind and prewash (at least when I'm working with white)