I only prewash fabrics that might run these days. I test them first before going to that trouble, and it's usually the reds and the bright pinks that do. I just iron the backing fabric after buying and before delivering to a long armer. Even though you may have washed the top fabrics to avoid shrinkage, the finished quilt will still shrink a little because the batting shrinks. If the back shrinks, it will most likely be just a small amount.