I would not bother to prewash the muslin as long as you are doing a reasonable amount of quilting. (This would be more than SID around 12-inch blocks!) What happens when you quilt is that the top and backing become tied to the batting. Once that is done, the batting controls shrinkage. Even though a fabric might shrink more than the batting if the fabric is washed on its own, it will not shrink more than the batting once it has been quilted to the batting. The only rule is that there has to be enough quilting to actually ensure that the 3 layers act as one. A large stipple is enough.