I leave my fan on at all times, personally, just so that the plants get fresh air (e.g. air containing co2). The plants consume the co2 in the air they 'breathe', so keeping the air moving is good for them, just as it's good for miners to have fresh air for the oxygen they need. The only time I turn off the fans is when someone needs to enter the room for some reason, and I don't want to advertise what's going on.