i wasn't up to doing any sewing last night so i decided to read my Complete Guide to Quilting and i read about using bed sheets for backing and this is what it says in book. I'll quote from book:
TIP: Bedsheets for backing- don't do it!
Although it's tempting to use a bedsheet because of its size, sheets don't make good quilt backs. The thread count is much higher in sheeting than in common quilting fabrics. The tighter weave causes the needle to break the sheet's threads when it pierces through, rather than pushing between the threads as it does with quiting fabric, thus leaving holes and diminishing the stability fo the sheet.
I haven't finished a quilt to do any quilting so i don't know how true this is, i guess its up to the individual if they choose to use bed sheets. I just thought i'd pass on what i read last night. Good luck with your choice.