Well for FPS with up going to look down felt more natural.
When I move my upper body or head forward I look down, not up.
Hmmm... this might be the case why I get confused with FPS setups like that. For me it's not mouse up but forward
![:o :o](https://www.systemshock.org/Smileys/sbf/wot.gif)
But this gets me in trouble regulary because there is always some game which doesn't support inverting the axis. Especially on consoles.