Considering how low the dollar has dropped against the yen, it would be best to try to save as much as possible on the exchange rate. From what I have read, it appears that the best rate is given in the Narita airport. Their current rates are updated daily on this website:
http://www.narita-airport.or.jp/exchange_e/index.html
Take note that you get a better rate exchanging traveler's checks rather than cash so do the math before you go and figure out whether the fee for purchasing traveler's checks still gives you a better deal than a straight cash transaction. It appears that there is only .025 USD difference cash v. tc so unless you are exchanging thousands of dollars the rate advantage of a tc v. cash is relatively nominal but you still would have the "security" of tc's.
http://www.narita-airport.or.jp/exchange_e/index.html
Take note that you get a better rate exchanging traveler's checks rather than cash so do the math before you go and figure out whether the fee for purchasing traveler's checks still gives you a better deal than a straight cash transaction. It appears that there is only .025 USD difference cash v. tc so unless you are exchanging thousands of dollars the rate advantage of a tc v. cash is relatively nominal but you still would have the "security" of tc's.