Jeanne49 mentioned not using sheets on quilts for show but guess what? If you use a sheet it is very unlikely the judge or anyone else will ever know. I have been complimented on my backings many times by judges and other quilters and they have been sheets! I think they probably thought that they were wide quilting fabric backs. I think that using sheets is just a "quilter bias" and it shouldn't be an issue. If I need the size, I open the hems of my sheets and if I don't, I cut them off.