I have used cheaper white or off-white cotton on some special white/off-white blocks to prevent shadowing. If I am using an off-white in my quilt (like Kona snow) I generally use an off-white batting. That helps a lot. Likewise, if I use a true white in my blocks, I will either line those blocks with an additional layer of cheaper white cotton and/or make sure my batting is actually white. I don't use muslin, as I have found the coarse weave of a lot of muslin for backing has shown up at times through the white fabric in the block.