I go lamp on.. Mostly cause I have to do it that way in flowering anyways so I don't hermie my plants when opening the door.
Depends on what stage; veg or flower. If you are in flower you have to go lights on unless you have infrared vision and can water in pitch blackness. In veg you can water whenever you like.
IMO its best to water when lights first come on to give it the day to dry up a bit on top of the soil.. I say this because DARK AND DAMP=MOLD and moldy soil is no good... choke with ya later...