I have wondered about striping the backs horizontally, since while I am using the quilting frame, those seams are taking quite a pressure. It is pretty amazing how tight you can get the fabric when you are anchored at one end and have a ratchet style catch on each end of the cross pieces.
So, when I read "only vertically", it made sense to me ... but then they started to using all sorts of seams to make the backing, and when you realize the top (which has all sorts of seams) goes through the same pressure ...
all of my reservations disappeared.
Probably the most important thing to remember is the stitch size as you are doing the seams. The bigger the stitch length? the less tension it can handle.