For aesthetics, seams should be hidden, which means on plain square and on custom round elbows (made out of 2 pieces of pipe rather than having the incremental bends),
the seam would be on the back, against the wall, versus it being visible.
This is not an option for regular elbows that are made with incremental bends, such as square corrugated and standard round elbows as the seam would have to be stretched
if bent the other way and would break.