In Club on nighttime flights I have my seat flat and off to sleep about 2 minutes after take-off, well before the seat-belt sign is off. Absolutely no need to undo the belt so I think I'm complying with the rule. However I'm a bit superstitious about taking a safety risk that I might not live to regret so I always wait until the engines pull back from max power before doing so, I don't want to find myself fast asleep in bed in a emergency landing situation if the plane has to abort the flight, just for the sake of waiting one more minute.