I have used flat sheets for the back of quilts, with no problem at all..there are a couple charities i know that use the flat sheets from walmarts..it is one of the few places that sells them seperatly.
you can pick up christmas flannel sheets cheap now..i have used them on the backs of Christmas quilts..
i am not a purist, and no quilt police are allowed in my home..:) my quilts are not junk either..I also do alot of charity quilting..