Pretty much every quilt I've made ( hundreds) has contained prewashed and unwashed cotton fabrics. I've never had a bad thing happen because of it. Generally I prewash if I'm using deeply saturated colors of hand dyes or batiks that might bleed, if a fabric smells funky or feels funky or if the weave seems kind of loose & I want to tighten it up. If none of those issues are present I don't bother. I always wash and dry my finished projects. I consider it the final step after binding