I do my own long arming. I piece my backs with a 1/2" seam ironed open. That just seemed the right way to do it for me. I'm glad to see others take the same approach.
I loaded one quilt with the seam vertical and by half way down the quilt, I had a bigger mound in the middle and had to work to keep everything lined up after that. I will no longer load a single seam backing in any other way than horizontal. Sewing over it one time is not a problem, and horizontal causes the quilt to stay loaded with the same tension all the way across the quilt. Now, if the backing has several seams or has quilt blocks in it, then you don't have the same type problems. There's extra seams here and there to keep the back in balance.