A lot of professional long armers won't take a sheet as backing because of the high thread count and the difference between the thread counts in the top and the sheet backing. Sheets are harder to quilt and dull needles faster. I have used a light muslin for batting for a light quilt, but not yet for backing. Having said that, I have come across muslin that I thought would be suitable for backing, but find it too plain to be pleasing. I prefer to dip into my stash for backings, and either I have enough of a fabric to make a backing (out of the same fabric), or I do a pieced backing. Just my thoughts on the matter.