My observation based on my own bike and personal geometry revolves around the heigth of the handle bar compared with the saddle. If the bar is above the saddle a bit, then I find the bar more comfortable if it is tilted up and back a bit. During times when I'm very fit, say around mid summer. I like being stretched out bit with the bar a bit below the saddle. In this case, I like the bar tilted down a bit as this provides a better angle at the wrist.