you can be within 2" of the plants with fluorescents or CFL (spiral twisties) and not burn the plant. If you are flowering, be prepared to move the lights up daily for the first 2 weeks and do your best to not burn them in this stretch period.
If the plants are not touching the lights you should be fine. Like stated above, when I grew with flourecents, mine grew right up into the lights and didin't burn. Also like stated above, a ballast can get hot and it can transper it's heat through the metal backing and is more likely to burn the plants than the lights themselves. Good luck!