I feel the best squeeze in my back when I bring the handle to my lower sternum. I'd say whatever gives you the best squeeze, and feels best.
I also alternate grip to switch things up: Close-grip palms in, wide grip palms in, wide grip palms down. Personally, palms up hurts my wrists so I don't do them.
I think there are many right answers, just use what feels right and don't settle on just one.