You should put them on charge for a little bit before heading out. And idle battery slowly looses it's charge over time. But you don't need a long charge time, just pop it on the charger before going to bed the night before flying (unless you stay up until 3am, and leave for the field at 6am the next morning, but you get my point).

You CAN hurt the batteries by overcharging them. Which is why you don't want to just leave the batteries on charge all the time. However, with the slow charge rate of the wall-wart charger that came with your radio, you actually have to charge it for quite a while to do any damage. A bit of extra time on charge won't do anything. I've accidentally left batteries on charge for 3 days with no harm, for example (don't try this at home, I got lucky, you may not).

One cute trick some guys use over the winter is to set their wall warts on one of those plug-in timers used to convice theives you're actually home. Just have your charger turn on for about an hour each day. Fully charge the packs, then put them on the timer, and they should be ready to go when you are. You can also buy trickle chargers that are designed to maintain a charged pack at full charge with out over charging.