I've been making scrappy backings with almost all of my bed quilts and throws that I've made in the last year. It's a great way to use up the scraps of fabric that you loved when you started the project but have had your fill of in the meantime. It helps control the size of the stash. Basically, there are no rules when I do this - I sew, trim, sew some more and see what I end up with. However, a friend of mine had a great idea, when she pieces the back, she'll do it whichever way works except that she'll usually make an extra row of the blocks and runs them along the top of the quilt. That way, when it's on the bed, if you just want to turn down the top of the quilt rather than tuck everything in, it's still decorative and matches the quilttop.