The OTHER side to pre washing (besides shrinking and bleeding) is that your batting also has a percentage of shrinkage. The package tells you what it is. If the fabric shrinks a little and the batting shrinks a little and the backing does roughly the same, well you (I) won't have too much of a problem making the item flatten out, right?
Right now I'm making baby things and backing them with pink, blue or green flannel. I even dry the flannel in the dryer - something I don't do with other fabrics. Right or wrong (correct me if you know) the flannel will be the worst shrinker of all my quilting fabrics.