On a 4.8 volt Rx pack I recharge when it drops to 4.7. On a 9.6 volt Tx pack I recharge at 9.5 volts.At 4.8 volts a pack is performing at it's rated capacity so it's good for several 10-12 minute flights. There is really no reason to charge them until it gets below that.
I used to have a phobia about batteries and overcharged them all the time. The battery life was significantly reduced. Since I have started running them down a little lower before charging they last a lot longer and hold their charge better. I don't use 6 volt packs so I won't comment on them.

WCB