Charge as many as you need but use them once charged, its not a huge deal if now and then you leave one charged but do not rely on the auto discharge function too much. The smarts and circuits we talked about are not fool proof and if they fail for whatever reason (a good one is that the circuits are on a flying machine and these can take some hard knocks) then it wont cause a fire but it will shorten the battery life, a common failure of a battery is a single cell in a battery dies and from there the battery is finished. Think of these batteries as a rubber band which is fully stretched at max capacity, it should not stay in that state for too long. At the same time they also should never be fully flat, 3.8 volts per cell is what we call storeage capacity, max volt per cell is 4.2 volts, if you keep draining the cell to below 3.2 volts per cell or so and it wont last long. You dont need to worry too much about these numbers as long as your batteries work right but its good info to know.
Thanks Chavezshutter , that's really good information to know, I didn't knew about that. So example I can safely charge them before leaving them to idle for a couple of weeks, and when I decide again to use them, I need to fully charge them again?