I don't use screen protectors on any of our iDevices. Not even on the kid's iTouches.
Gorilla glass has a hardness of 9 on the Mohs hardness scale. The glass on our iPads is basically has hard as a sapphire. If you remember from your middle school science class diamond has a Mohs hardness of 10. Regular glass has a Mohs hardness of about 7.
Unless you use your iPad at the beach I wouldn't sacrifice the clarity and smoothness by adding a screen protector.