I only mount horizontal seams on my LA - but sometimes I rotate the quilt if I have a pattern that I want to run a specific direction. I also sew cross grain to long grain when I'm using up extra wide backing scraps. So far I've not run into any problems doing this. I'll sew shorter vertical (2-3' long) and that does not cause a problem. I used to be real particular, but I really wanted to get the backing scraps used up
I've been using up scraps the last several months, both for tops and backs. Sure is nice to clean up all those cut off pieces of backing.
BTW- my machine likes the top to be quite loose, so there is no tension on the fabric when stitching. I think that takes care of a lot of potential problems. With my first setup, that wasn't a true LA, the top had to be rather tight and mounting the backing sideways caused grief. I like being able to take the quilt off and rotate it for doing custom border designs.