It depends on how long you will be in Tokyo and how much you plan to ride the subways.

Subway tickets within the metro area start at 170 yen and go up to 310 yen. (The subways and private rail lines are more useful in Tokyo than the JR lines are, but you can use the Suica for both.) Some of the subways turn into above-ground trains in the suburbs and cost more, but you are unlikely to go to any of these places as a first-timer.

I generally start with ¥5,000, but you if you run low, you can add value at the machines found in every subway and train station.

If Tokyo is your first stop, don't validate your JR Pass until you are ready to leave and just use the Suica for everything.

On my 2012 trip, I arrived at NRT one evening, paid cash for the N'EX to Shinagawa, spent the night there, and got up early the next morning to validate my JR Pass and take the first leg of a trip to a conference in Hiroshima. (As a passholder, I couldn't take the fastest Nozomi train directly to Hiroshima but had to change at Shin-Osaka, which involved standing on the same platform for all of 10 minutes.) After the conference was over, I used the Pass to tour the island of Shikoku and visit some clients in Osaka. Then I headed back to Tokyo, where I used my Suica to take the subways and other trains to various places in the metro area and the remaining days of the JR Pass to take two long-distance excursions northward. Finally, I took the Skyliner back to NRT.