I love to use up the leftover fabrics from the top on the back. It's cost effective and just makes sense to me. I don't love saving leftover scraps that I probably won't use in another quilt project. I don't always make a design, but try to use similar sized pieces. Sometimes I have enough for the middle of the back and then I add a coordinating (almost solid) around the edges to make the back large enough for the top.