Not related to the original question of checking in the day before, but itinerary wise, what happens if OP's EWR-NRT flight is late and he misses his NRT-GUM flight? Since there is only one flight NRT-GUM, I'm assuming OP would be spending the night in the connections area of the terminal.

Back to the original question, I add that checking in at the airport the day before is a good idea. Also, on departure day I would get to the original airport early as well.