I was machine quilting just this week and had the backing seam running the length of the quilt as it was mounted on the frame. My quilting mentor told me it's better to have it run horizontally on the frame since the seam doesn't stretch as it's being quilted (unlike the rest of the backing which will give a little as it's quilted). So if you're machine quilting, that might make a difference in how you plan your backings - it will for me :-)