what light are you using is the key question...if it is a cfl you can get it as close as possible to the plants...but if its a 400 watt hps bulb (or something similar) you can put your hand above the tops and the lower the light until you can feel heat on your hand (point being that if the buds are warm from the light they are too close)
but at the end of the day this is my preference, and is my safe-haven for growing...if you would like to experiment with closer lights by all means go for it and let us know if it increases your yields!
good luck and happy growing!