I keep my 250w HPS about 6 inches away from the top of the plants but only because I have a fan blowing directly at the bulb. If there was no fan to cool it down I'd have to keep it a few inches higher.
When I use my metal halide bulb I keep the lamp at least a foot away from the plants even with the fan on because the metal halide gets much hotter than the HPS for some reason.