For what it is worth, I think this is a matter largely of your seating combined with personal perference. The height and angle of your seating gives you a natural head position that should help you decide where you can most naturally see without neck strain. Whether your eyes look strainght ahead, up, or down from there is a personal preference. You may also find the ability to adjust the display's vertical angle effects this decision including the effect this may have on reflected light from the room.
Cheers,
Gary