I grip as high up as I can, and shoot with my thumbs forward. It's an XDM 9 4.5".
At first I thought it was just bad form on my part or that I was letting my thumb move a bit during recoil. But I recently read a description of the grip used by Bob Vogel (a world champion pistol competitor for those who don't follow such things). He has the same issue. I'm not claiming to be Bob Vogel by any means, and I'm not trying to be a fanboy. But I figure if a shooter as capable as he is has the same problem, then there might be more to it than just my grip.
So I did some reading, and some other people people are having the same issue with the XDM specifically. I think it's because the slide release sticks out a little farther than the XD, and (it seems to me) the slide release is a little closer to the grip. It's the only full-size pistol I've owned where I could reach the slide release with my strong hand without shifting my grip a bit.
"An armed society is a polite society." — Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
— William S. Burroughs