I also use cotton sheets and cotton flannel sheets for quilt backings. I just make sure that they are 100% cotton with a thread count of about 200. I buy a lot of my sheets from thrift stores. If they feel good and look good, especially the flannels, then they are "fair game" for a quilt. As a hint, look for duvet covers. There is a lot of yardage in them. I bought one last week for $3.50 (senior discount day ! and already marked down) that measured out to be 12.5 yards of fabric, cost 27 cents a yard.