I can play either way with 3rd Person Shooter. If the 3rd Person Shooter goes into First Person Shooter mode, eg. Jedi Knight, the I will prefer to play normally.
But not on first person shooters.
With FPS, if I want to look down then I will move my mouse down.
But with 3rd, it's not that I am looking down, it more like I want to move the camera angle up so that i can look down.