A couple of late observations...

If you know which trains you want to take (ObHyperdia), write them down and take them with you to the JR office at Narita. When you pick up your pass, the agent can make your reservations at the same time.

Remember, it's by day and not by trip, so on the days you use your pass your local JR travel is also covered. And, of course, your tickets are not for use in the magnetic readers... manual gates only!