As iamfred said, too many charges is bad too. The kindle automatically turns off before the battery discharges too far (unless it is bricked or stuck in diagnostics). The best usage for longest battery life would be to NOT charge the kindle until it shows low battery, or tells you to charge it... But do not let it stay too long at the "needs charging" screen either.
For long term storage, it should be between 40% and 70% charge for maximum lifespan. Both too little and too much charge is bad.