I have used washed and unwashed fabrics, and have never had a problem with uneven shrinkage. Although I think it is best to use all one or the other, it just doesn't seem to work out that way for me many times, guess I am lazy. But I NEVER prewash precuts like charms or jelly rolls, and if I want to use them with other pre washed yardage from my stash, I do so.