Over the years I've gone thru dozens of 150w HPS bulbs. Not because they burn out, but because I replace them after every three or four grow cycles. A new bulb is considerably brighter than one that has alot of hours on the meter, so to speak. I've also used them in every possible mounting position and never lost one as a result of its orientation. Add all this to the fact that I buy the cheapest bulbs I can find, usually at flea markets with little packaging or info, and I really think that its a non-issue in the 150w HPS world. I know its a real concern with larger wattage bulbs and those using other HID technologies, but for us, I say just mount it so it gives you the best possible coverage. JMHO, Regards to all, PD