I usually do this "floating" thing, especially since I mostly use my index and middle fingers, so I have to move the hands around to reach the keys.

But I've found that for some use cases, when I always press the same keys (say moving around text with hjkl, or similar) it's very comfortable to rest the wrist on the wrist-rest. However, I usually put most of the wrist's weight on the exterior, so I suppose the nerves aren't as much constrained.