I piece all of my backs. I use what is in my stash. My stash is out of control and I am trying to avoid buying any fabric. I usually make a back that is not a straight design. Put something purposely off center. That way when it doesn't end up perfectly straight on the back of your quilt no one can tell. Also if you do all the same size block when you trim your edges it will show. I often loosely copy a simple modern quilt for my back. google quilt backs or modern quilts for ideas.