Is it better for the battery to leave the iPad plugged when charging is done?
It doesn't really matter. You're already using up some capacity when you charge past 50%.

For long term storage, Apple recommends that you only charge the device up to 50% and then power it off. Basically if you are concerned about the longest capacity of your battery, you wouldn't consume it at all and you'd just charge it to 50% and then put it in a box for 6 months as it slowly self-discharges. You would then power it on and charge it back to 50% and put it back in the drawer off.

Realistically, just use the iPad and follow the instructions Apple gives to the letter. As long as you disconnect it from charge and regularly deplete the battery you won't have meaningful problems with excess wearing. As long as you're not in a long term storage situation, disconnect the cord when it's convenient to you.


http://www.apple.com/batteries/maximizing-performance/