It's not worth playing FPSes without a mouse, so I use normal for them naturally. The thing is, I sometimes run into a problem with normal/invert in third person games.
For third person games that use dual analog control, I prefer inverted controls. However, when I go into first person sniping mode in a third person game, I'll sometimes get confused and expect normal movement instead of inverted. Certain things can mitigate this effect, for instance a zooming in animation when I go into first person view can do a better job of keeping my brain oriented to the inverted scheme.
It's pretty annoying when this kind of mental disconnect happens in the middle of a frantic situation. :/