Hopefully a powerlifter will come in here and prove me wrong. Since I have little experience with max lifts, I may be way out in left field here.
However my thoughts are, since you are going for maximum pressing strength, then when you choose your grip, you should pick a grip where both the primary movers are in an ideal position to press maximally.
Take a look at your forearms as you are pressing, they will be perpendicular with the ground, leaning in, or leaning out. From what I understand, the idea is to pick a grip where your forearms are perpendicular to the ground for the greatest amount of time, throughout ROM.
So, they will be slightly out at the top range, then perpendicular as you lower the bar, then leaning in as you reach your chest.
Does this make any sense?