Sometimes I piece the backing because I have to (fabric is not wide enough), sometimes because I don't have enough of any one fabric to cover the back, and sometimes because I want to use up some stash. As a result, I wind up with many different layouts on the back. Since I longarm my quilts, I try to have most of the seams running in one direction so they can go parallel to the longarm bars, but that doesn't always work out. So, don't worry too much about "how" you piece the back. Just be aware that the seams or designs on the back will probably not line up perfectly with seams or designs on the front, and that may affect your quilting.