I run fans just to move air around and help make sure that humidity distributes through the tank. My large tank was without fan for a year and I didn't have any problems, but humidity was around 50%. With fan it went up to 55-60 %.
I don't find it necessary in my conditions to run it 24/7, but I don't have misting or drip system, and water by hands, so there is not much threat of a fungus or other nasty problems that would come up without fans 24/7.