I've had to add left over blocks of fabrics from the quilt to make enough backing when I've run short. I've also read that it is actually a good practice to do this just in case the front gets damaged. The backing can be used to help repair the quilt. Fact or not, it sounds reasonable.

I don't like scrappy backs or backs with blocks. After all the work on the front, I want the back to be pretty and have the quilt reversible if desired. Just MHO.