I have all but two of my tank lights on timers now. I can still turn a tank's light back on (or off) if I want to and sometimes I do, but about 95% of the time I just let the timers switch the lights on and off.
On the two tanks without timers, I end up turning the lights on and off at times varying by hours from day to day. I don't think it's good for the fish and I need to put timers on those tanks too.