I do whatever works. I prefer the backing is the same as or coordinates with one of the fabrics on the front. Sometimes I have enough fabric left, sometimes not. I prefer prints rather than solids - just personal preference. I usually piece my backs. I prefer the seam lengthwise and not in the center, but I do whatever the amount of fabric I have dictates. Sometimes I do scrappy backs with leftover blocks and miscellaneous pieces from the front. It's your quilt. Do what works.