I can't think of a logical reason why they would tell you not to prewash. I wash all my fabric 99% of the time, the only exception is fabric from kits, which I seldom buy, as I like to use my own color/fabric choices. If you don't prewash you run the risk of ruining a finished quilt when it was washed, unless you plan to dry clean quilts. I agree with Lori S. and I would not follow their instructions, but it would be interesting to find out why they say that.