I never pre-wash pre-cuts---but I often pre-wash yardage---I've made literally hundreds of quilts- I have never had any adverse effect from mixing pre-washed and not pre-washed fabrics- unless I had a fabric bleed- and I tend to test any that I think may run & avoid that-so I see no reason to worry about it- there are so many factors involved in any shrinkage- including the batting chosen, how a quilt is quilted- how it is laundered----but most of my quilts contain both washed & un-washed fabrics. they turn out just fine. launder just fine are used just fine.