Even when fabric is prewashed a finished quilt ( might) shrink some too, there are lots of factors involved with if or how much a finished quilt shrinks. The backing, the quilt top (design/ technique plays into it too) the batting, and the amount/ type of quilting all play a role. So, don't think because you prewashed it will not shrink again, and on the other side, if you do not prewash your quilt might not shrink.... Many people make wonderful quilts, without prewashing and they are fine after being laundered too. Many choose to always prewash, then get upset when their finished quilt shrinks some ( cotton always shrinks, think of those favorite Levi's, shrink, get tight when laundered, loosen back up with wear- so will cotton quilts) some people choose to prewash because the fabric has an odor, or feels stiff, without considering shrinkage.... It really doesn't seem to matter much what you choose to do. I know people who will wash flannels 3 or 4 times before using them ( and then complain when they pill, or are worn out) and people who make wonderful flannel quilts without prewashing that handle laundering fine when finished. I usually tell people, don't sweat the small stuff, if you feel you have to prewash- knock yourself out- if you don't feel you need to, don't bother, either way your quilts will be fine, as long as your seams are secure and you quilt the required amount for the batting you are using.