I haven't been into quilting but about 7 years and have NEVER EVER used a sheet for a backing. I have been told by National Teachers that really good cotton fabric and cotton sheets are not woven the same. The cotton is different and when stitched the needle will split the cotton causing it to break and you not even know it. After you put lots of work into the tops why would you not want the best you can get for the back. If you watch sales you can get 40% or more off and then buy your backings instead of risking the life of hard work.