Flip a coin. Heads--angle, tails--butt (of course) . If you like the process and result, great. If not, you can do it differently next time. (But I'd probably be consistent within the same quilt.)
I do know from working with page layouts that the eye tends to starts in the upper right corner, and travel down and around clockwise. So if you don't want the seam to get noticed right away ,put it on the left side. If you have multiple strips joined, then stagger them.