You really won't do your iPad or iPhone batteries any good by undercharging them. I'm not sure if they have a "memory" affect from undercharging them. The Amazon Kindles used to require a cold boot and reset if you undercharged them.

USB's are great -- but I'd use your inverter with a proper charger; or, alternatively, a cigarette charger which is sized for the biggest Apple device. (The iPhones take will work well with the iPad's stronger output, BUT not vice versa).