It depends, for me anyway, on the print. If it's something like a solid, tone on tone, blender, etc I will use a diagonal seam. If it's a stripe, plaid, or big print, I might use a straight seam because a diagonal makes it more noticeable to me. I'm just anal enough to try to match up patterns and stripes and such, especially on something as big as a border. Probably leftover from when I made apparel.