I prewashed everything one time because I was taking a class and it was part of the instructions. I haven't and won't since. I ended up using so much best press and it never did iron out as well as unwashed fabric. I prefer to wash the completed quilt with a color catcher. And just to be really scandalous, if I'm just making a quilt for fun and have no intended recipient, I still might not wash it even after its finished. I have several that I've just folded up and put in a closet! My hobby, my rules.