I don't prewash. I like how well unwashed fabrics work. I also really like the "shrunk, old fashioned" look of my quilts after I wash them when done. I use cotton batting and it all kinda shrinks together and I like the look. I wash everything before it goes out the door and I use color catchers. Have not had an issure with colors runnin at all. Then again I don't use many dark reds. I will, however, wash all my flannels before using as they have such different shrinkage rates and shrink a lot more than "normal" fabrics. I tend to buy only the better quality (LQS) quilt fabrics, had several issues in the past with the cheaper fabrics and don't wish to repeat that. I am fortunate as the starches in the fabrics do not bother me.