I use sheets all the time, all cotton, cotton/poly and flannel with great results. I make scrap quilts and they often contain older fabric and I can't justify spending $15 a metre (at most LQS in Canada) to purchase fabric for backing. My quilts are practically all given away and expensive backing fabrics would make this impossible. I have had absolutely no trouble machine quilting through sheets on my Janome 6500, 201 Singer or Husqvarna Viking#1 Plus. I buy the largest sheets that I can find when they are on sale. I often buy seconds at the Lens Mills stores or Sears or Giant Tiger and they are usually less than $10. My guild also uses sheets for the backing on our Community Quilts. The only problem that I've ever had with my sheet backings was one sheet (probably had a higher poly content) pilled after frequent repeated washings. By that point the rest of the quilt wasn't in good shape either. If I have a King sized sheet for backing a smaller quilt, I make a matching pillowcase from the left overs. Sheets definitely work for me and I don't care what the quilt police say on this one!