Lowered! So you notice what's above you when looking forward, and notice what's in front of you when you look down.
Centered is claustrophobic and there is no reason what so ever to cover half of my screen with floor/ground, I'll have a reasonable amount of ground pls, and some grass with that.
In Halo 5 I completely missed the sky-boxes. In CE it took me a few playthoughs before I saw the life-pods in the sky on the mission Halo, and Cortana literally says "more life-pods coming in", not that I would be guaranteed to see those with a lowered cross-hair or anything.
In Halo 2 when the In Amber Clad comes into High Charity is an example when lowered cross-hair makes a difference in campaign, ofc the extreme vibration in the controller helps too.
In Halo 3 when you approach the mac gun on The Storm, it shoots down a Longsword. I would never have seen this with centred cross-hair.
In mp centred cross-hair also limits the awareness around you, I don't wanna be dependant on turning the FOV up and make my image misshaped and ugly to play better. And imagine how much ground I would get with centred cross-hair plus high fov, I don't even wanna think of that, ground overload, I'm fainting.
There was a purpose with lowering it in Halo 2 and onward (it even got lower and lower in 3-reach-4. Other games even followed Halo's footsteps because it was revolutionising in a very small way.
The one only point I of having centred that I can think of is basically symmetry... Which has never been important to me.
Option is the best solution, let us choose our preference.