I find if I'm completely verticle I come too close to bumping my chin with the bar.... and therefore I get some shoulder joint pain from holding the bar too far in front. Luckily my gym has a seat which leans back only a few degrees, so it's practically verticle but enough so I don't lose my nose.
Just make sure it feels natural for you.... your tendons and ligaments will line up in the natural way, and it's not good to be pressing outside the natural plane of motion.