I usually wait until the top is finished before figuring out backing. Most often I take the finished top to the LQS to choose the backing. My stash is predominantly smaller pieces so I can't 'shop' for backing at home. If I'm making the top from a coordinated fabric line, I sometimes buy a backing from the same line at the outset. I like both pieced and single fabric backings, depending on the quilt. I also love the look of a plain muslin backing, when appropriate. The stitching shows up so nicely on a solid back.