I've often pieced down the middle and never had a problem with my backs. How I do them varies by the fabric I am using. If the fabric is busy enough and I only need it a little wider, I've added a strip to one side. You can't even notice where the seam is! Most times, I make a decorative back. It's a great way to use up fabrics from the front and adds a little interest to the back. Sometimes I add blocks, other times I add strips. I have pieced the whole back with large squares, or chunks of fabric. I do tend to like things more symmetrical, so I will often add narrow borders between a row of blocks and use that as a separator for 2 larger pieces of fabric. I have taken a full width of fabric and just added borders to make it the right length/width. I sketch out what I'm thinking of doing. I lay out my fabrics and play around with them until I like the combination.