It depends whether you have plants or not. If you have plants, it's important that they get a resonable amount of light for their survival, although some need more than others. Between 10 and 12 seems to be the general consensus. As long as there is light but it is not on all the time, fish need night too, then it should be ok.
I have all my lights on a timer. My 2 smaller tanks have white lights on from 10:00 to 22:30 and my biggest tank has white lights on from 10:00 to 20:00 and then a dimmer blue light from 20:00 to when I go to bed.
