I've done a lot of diagonally-pieced backs, partially to avoid the lumpy seam on the bar, and partially to get a wide enough backing from 44" wide fabric.

I prefer my wall hangings to be backed with the long grain vertical. With bed quilts, I don't worry about direction so much since I do verticle, horizontal and even diagonal seams to save farbic where practical.
I do splurge and try to make my outside border lengthwise to stabilize the quilt since keeping the front stable is important to me.