I have pieced backings lengthwise, or added blocks down the center, and other ways to get the dimensions needed.
Some who LA, prefer that the seam does not go vertical, as it causes problems rolling them on their machines.
I quilt my own, and will piece the backing according to who is getting the quilt, my whim at the time, and by the backing that is readily available :wink:
It is really a personal preference as to how you do this :D:D:D