Helps to leave fans running constantly, yes, but they don't have to run during the dark hours.
I don't mess around with PC boxes or fans or power supplies either, but as tet1953 said everything has some limit to what kind of load it can handle. Unless you're running something particularly demanding though you should be alright; fans don't generally draw much power, but the wiring needs to be correct and it should be capable of handling more than the intended load.
Timers for example: your common 'general purpose' mechanical or digital timer is usually rated at 1000 watts. Say I wanted to run a 1000 watt HID on this timer; not safe. I would really want a 'heavy duty' timer for the purpose because the bulb and ballast is actually going to draw 1100 watts. If you absentmindedly did this and to top it off decided also to use that single timer for fans, pumps or what have you- you'll definitely be way over load.