I only wash fabric that I think will run after the quilt is completed. Like the reds and some saturated blues, etc. Generally I don't like to prewash because I lose too much fabric. I tried cutting the corners and also cutting the edges with pinking shears but I still lose a lot of fabric--I hate losing 1" of fabric at today's prices!. I have never prewashed fabric that didn't unravel. However, I am scared that the bleeding reds will spoil the finished quilt.