Of course you can use muslin, but know that every stitch shows on the back of the quilt. I mostly use prints on the back of the quilt, so it hides the stitches, the starts and stops. And it doesn't show the wear as fast. If you do beautiful quilting, then maybe you want your stitches to show, and muslin or another solid color would be a good back for you. I don't recommend sheets due to the high thread count.