I've actually never used muslin on the back of a quilt, although I have used it on the front and I do think it can be a nice backing, depending on the quilt. On scrappy quilts I often try to use up the leftovers from the front. I very seldom buy a fabric just for the back, but when I find fabric that I really like at a REALLY good price I buy a bunch of it and put it aside for backings. One thing to think about is that your quilting stitches show a lot more on a solid back. If you want to hide them, use a print; to highlight them, use a solid.