I use sheets all the time for my quilt backings. I stock up on King size flats when I find them on sale (often less than $10) Canadian for 100% cotton. I also use flannelette sheets for my warmer quilts and my kids think that they are especially cuddly. I like the King size ones because they are often only a couple of dollars more than the doubles and the extra fabric can be used to make pillowcases. Someday I may make an heirloon quilt and want matching fabric but for now co-ordinating sheets match my budget and the hard use kids put their quilts through.