[Sangetsu]

Bring the cash. You'll be able to use your ATM card here (in some places), but you will be dinged for an international transaction each time. In the long run, it will cost you more to use the ATMs than it will to exchange your money. You can exchange your money in your home country before you arrive here, do a search for currency exchanges until you find the one with the lowest service fee.

No business in Japan accepts checks, travellers, or otherwise. You'll be able to exchange them at banks, but rather than go to the trouble of buying them, and then exchanging them for cash, just bring the cash.

You will not be robbed in Japan (unless you like drinking in hostess clubs), and almost everyone here uses cash for every purchase. Convenience stores and fast food restaurants always have change for an ichi-man yen note ($100 bill).

I usually carry between go-man and ju-man yen ($500 to $1000) in my wallet at all times. I've never worried about getting my pocket picked, or being mugged.