I really don't concern myself with how the seams go on a backing. When I get the build up of seams running perpendicular to the rails, once I load the back onto the rollers I find if I roll them back and forth several times (before putting on the batting and rolling the top, I don't float tops) it takes care of that looseness on the sides that can happen. I tend to piece a lot of my backs, to the point that the quilt can be considered double sided, like this one (
Triangle quilt ) so a lot of seams on the back, no matter what direction they are going in, don't bother me.