I think this question is quite a good one for those that perhaps only wear their watch at the weekends or when "going out" somewhere nice, rather than as an everyday watch. In this case, it will shorten the battery life by a small amount leaving it permanently on charge (which as mentioned is highly controlled). The best solution is to charge, and then power-off and put it in a watchbox or drawer. Even if you left it for a month on charge, I think the worst that happens is battery degradation but I would think no worse than if you had actually worn the watch that day and then charged it up overnight.