There are many ways to do the back. I've pieced down the middle with great results...no issues with stress on the seam. Usually I will add blocks or a section with a contrasting fabric and stretch my backing fabric, which is great if you're a little short. Some people put a seam down both sides of the selvedge, then cut one piece in the middle. This gives a fabric's width in the center and 2 narrow strips on either side.
I've looked into the diagonal method before and it required more thinking than I wanted to do. ;)