They don't need any water added for the last 2 days, but I choose to give my plants light each day until, well, you know.
It's been suggested that giving 48 hours of darkness before harvest will help them finish. I tried it once, and all I noticed was that the trichomes appeared to have ripened some.
Actually, it doesn't jive with the way I think about harvest. I believe that THC is developed during the daylight hours, so I always harvest at the end of the day of 12 hours light, and then I move them outside to sit in direct sunlight for several hours before I take them. I do my manicuring under the sun in my back yard.
HTH