I piece the back depending on how I will quilt the front. It is very hard to quilt a sashing line on the front of a quilt and have the quilting line line up perfectly with a center seam line in the back. I try to piece the back in thirds usually so this doesn't happen. I also have purchased the wide back fabric so there are no seam lines. I really like pieced backs and I think my next one will be after a design I saw from a link on QB. She had done two different colours on the back and a cool zipper design down the middle where they joined.