I mix washed/ unwashed fabrics in the same quilt all the time. I've never had any type of negative issue. I prewash fabrics that might bleed ( deeply saturated hand dyes) or if the fabric smells or feels funky. If none of those circumstances are at play I generally don't prewash. I make a Lot of quilts! I've never had something go wrong due to this practice. And, I Never prewash precuts