Left-right i.l.o. North-South. North-South gives you a larger (mentally) margin for error. Boradside (i.e. left-right) makes you pick a side of the spine and fire.
Wouldn't you have to
pick a side of the spine (unless you were aiming directly at the spine) whether he was facing directly away, directly at your tree, to the left or to the right as long as he was standing directly under you?