Leaving the device on the charger may shorten the life of the battery due to the potential of wasted charge cycles, but the effects won't really manifest themselves for a couple of years. Ideally, you want to take the device off charge when full, and recharge it when the battery gets to about 15%.
Leaving a battery on charge however won't damage it. In other words you can't overcharge the battery by leaving the device plugged in.