The traditional way to seam a backing fabric is to make two off-center seams rather than a single center seam. With regular width fabrics, this usually meant having a single wide panel width in the middle with narrower flanking panels on each side. The idea was that a centered seam in the back would present a single line of weakness in the quilt; they wanted quilts to last for decades through a lot of hard use.
These days many people piece patterns into the back. I have particularly liked those that simply have a diagonal slash through the back with contrasting fabric in the slash -- simple, but interesting!