I use unbleached muslin for backing most of the time. I hand quilt and muslin is so much easier to work with and I don't have to worry about seams on the back.
I bought white muslin for the backing on my double wedding ring and for the white areas on the front. Have also used the white for a SBS and a teddy bear applique.
I used a white sheet once for backing and hated it. It was too heavy to hand quilt easily.