I haven't read much on this, but I have been trying it, just because I've noticed the different ways my body reacts to warm vs cool water. The trouble is that I don't like standing up the whole time; crouching makes me very aware of heat being lost through my knees; and then I wind up wasting a lot of time and water with the heat turned up.

The obvious solution seems to be wrapping my knees in something, which I've done a few times with success; it's just hard to form into a habit.

So far, my experience points toward cooler water being beneficial, not that my anecdotes are anything resembling scientific (even as self experimentation goes).