It is discussed right here:
http://www.advancedaquarist.com/2002/1/review/view

You know, somehow I think that the bulb manufacturer's would know how their bulbs should be correctly operated...

manufacturers are defining the arc's burning positions as one of the constellations shown in the following picture:

If you look into a product catalog you can see these pictures and abbrevs. such az (V)BU = vertical - base up, (V)BD = vertical - base down. (the base of the lamp is where the edison thread can be found). so it not got me off guard when the guy figured out, that the iwasaki lamp which is a BH (base horizontal) performs better, when it's operatted horizontal. we allways ending up with this: RTFM

My very first config had the nipple pointing towards the lcd, this cause glowies in my black screen. I rotated the nipple straight up (towards the top of my box) and the problem dissappeared. That leads me to believe the nipple should be out of the light path as much as possible.

remember, we are NOT talking about burning position (hor/ver/angled), but nipple position (up/down/sided)

I´ve tried rotating the lamp, so the nipple would point a different side each time.
I saw no changes at all.

As long as you're using the manufacturer's listed burning position then I think it is more important to get the return wire out of the light path. If you're making a straight-shooter then this is usually at the top or bottom of the bulb in a horizontal position. If the 'bump' in the quartz is in the same orientation as the return wire then everyone wins.