I would never prewash a quilt top!
Also, there are lots of folks who do not pre-wash anything and back their quilts with non-shrinking fabric (sheets come to mind), and have no issues.
I back most of my quilts with flannel; and although I pre-wash the flannel prior to adding to the quilt, it still shrinks at a different rate than the regular cotton front, but there is never an issue or concern.
Once the quilt is quilted there shouldn't be an issue.