I tend to use muslin for the backing when it's a wall hanging or table runner, but for lap quilts or bed quilts I use a coordinating fabric. I'm just not sure how muslin would hold up with constant cuddling under a quilt. With a wall hanging or table runner, there wouldn't be a lot of handling of the quilt. Besides, I like the look of coordinating fabric for backing that will be seen a lot. JMHO