DON'T overcharge the cells. Use a good charging technique. DO choose cells wisely. Constant current charging is OK for the old 500 mAh AA cells and a charging time of 14 hours. This means that you need at least two sets of batteries and two chargers when you want to do two dives a day.
Trickle charge is used to keep a full accu fresh.
Use it over a few days, but not over months. It definitely increases charging time and puts some stress to the cells. It is no longer recommended to routinely discharge cells. Timed charging is what you usually get when you buy a lamp or an underwater flash that comes with a charger. After a given time, the charger switches to trickle charge.