I've used whole poly/cotton sheets for backing a couple of times. I wash my quilting fabric as soon as I get it home, so any shrinkage is taken care of before I quilt. Then I wash the quilt when it's finished to check that everything is secure and it looks good after washing.
So far, I've had good luck with the poly/cotton sheets. I also use flannel sheets that I purchase new at end of season sales in Sears. They have to be washed first, but work great, too.