Interesting timing. I don't prewash - ever. I've never had a problem. Tonight, I was getting ready for a class and it said you had to prewash and starch the fabric. I thought about skipping that, but didn't want a problem in the class so decided to wash. I found out one of the fabrics was a bleeder. I had to wash several times and then use vinegar to finally get the color set. Guess my guardian angel was looking over my shoulder.