I haven't used sheets as backing -- but I do watch the sales. I have 3 quilts that are very large king size quilt tops, and even a king-size sheet would not be big enough. At the same time, I purchased very good quality muslin (118" wide) and, using a coupon, got it for 60% off -- results - my cost $21 plus sales tax, instead of regular price of $52. Sheets can be pretty expensive, too.