It has always been recommended that the back be seamed in two places and not down the center. This makes it easier to straighten up the back. And always remove the selvedage to remove stress and possible stress. To add on either side makes it more difficult to keep the seams even up and down.
Check John Flynn's method. It is good for some widths.
However, I prefer to have one piece for backing. When you calculate the cost a wide backing may be about the same expense as using narrower fabric. The difference in time to align the back may be worth the difference in cost.