Sometimes I use leftover blocks from the front to put on the back; but, mostly I just piece it.
I always buy extra fabric in case I cut something wrong. I usually have enough left over to piece together a back.
BUT, I don't make up a 'pattern'. I just sew various 'chunks' of fabric together to make the back. I've never had a problem with my LAer quilting it. So what if it's a little cockeyed? What's to say I didn't 'plan' it that way?
Some of my quilt 'backs' are almost as pretty as my 'fronts'. And if I wind up with a little 'paw print' on the quilt after it's just been laundered, I can just flip it over... the quilt police (and Suzie Homemaker) aren't allowed in my bedroom! LOL