The iPad has very sophisticated battery charging electronics built in and can be safely left on charge indefinitely. The iPad carefully monitors the charge current and adjusts it continually depending upon the charge state of the battery. As you suggest, when it gets to 100% it adjusts the current appropriately. Also, iPad batteries do not suffer from the 'memory' effect that earlier generation batteries had, so you can also safely recharge a partially discharged battery without any ill-effect. Apple, however, do recommend you occasionally completely discharge the battery - not for the 'welfare' of the battery, but to re-calibrate the battery 'meter' that indicates how much remaining charge there is in the battery.
Tim