I recently received a packet from American Patchwork encouraging me to purchase their book. In it was several inserts of interest and quoting from one insert regarding using bedsheets for backing:
"Using bedsheets to back your quilts? DON'T DO IT!
Sheets have a high thread count, which forces your quilting needle to break the sheet's threads as it pierces through. This leaves holes in your backing and diminishes the stability of your sheet.
Quilting fabric has a looser weave, which allows your needle to easily slide between the threads and keeps your backing fabric in tact."
Just thought you'd like to hear their opinion. Suzanne