Prewash? Yes. Let's me know if the fabric bleeds, shrinks it, and gets out any "extra" chemicals and dirt/dust. Basically I do a rinse and spin cycle. No soap usually. Reds, dark blues & purples, blacks...all subject to bleeding, so those go in a hot water bath by themselves. I like working with clean fabric, plus it helps keep my allergies at bay. The only fabric I don't prewash is by Quilting Treasures, because they put thier fabrics through a hot water wash after all the dyes are in it. So if I can find the Quilting Treasures, I buy that to save the prewash.