I usually prewash, too, except for using precuts. Recently,when I was in a quilt store, I was speaking to the owner about mixing unwashed precuts with fabric I had already washed, because I was worried about uneven shrinkage. She told me that cottons don't shrink as much as they used to due to the production process, which is why fabrics have gone from 44" to 42" in width. I have combined prewashed and precuts in quilts, with no problems. (And I will always prewash yardage, just because I like working with it better!)