I am really lazy and hate to prewash...But I do do it sometimes! It depends on the colours I'm using. If black is involved then I will prewash because I've had a horrible experience with a beautiful quilt top that had a black border! I also think it depends on how I have bought the fabric. I sometimes wash fat quarters, but you do get a lot of tangley frayed bits. If I buy yardage then I will probably prewash it!