Palm. I wouldn't have guessed that anybody is holding the mouse in another way (I've only seen older people do so who don't actually are used to using a computer at all), as I find that to be highly unpractical and uncomfortable (even stressful for the hand). =0
Edit: Now that I think of it; when I move around with the mouse, it might sometimes change to fingertip (for the sake of not having to move the entire hand and arm). Or even claw for a short amount of time xD. I've voted palm anyway though, since that's the default-position of my hand.