i quilt for other people alot and many of them use wide muslin, bleached and natural; it is a nice fabric to work with- as long as you purchase good quality muslin. like with all fabrics not all are created equal. if it is very thin--yukky--i use it for foundations for (paper piecing)...
myself i use printed or batik backings on my quilts...then i don't have to worry so much about how the back is going to look when quilted; when using muslin or any solid every stitch shows ;)