Originally Posted by scirel
I'd agree with pretty much everything that has been said so far, and would advise you to avoid traveler's checks -- even international kinds or ones drawn in yen. Although banks seem to like them and will exchange them for cash, retailers have no idea what they are. They will stare at you as if you are trying to pay with a stack of leaves.
If exchange rate is what you're concerned about, stick with cash. Credit cards usually add on all kinds of fees which usually negate any breaks you'd get with low rates. Lots of info to be had on this site about credit card fees and international purchases...
It's the belt-and-suspenders approach, always have a fallback if choice A doesn't work. As far as travelers checks, I got a handful at AAA for no fee before I left, and the first thing I did at Narita after clearing customs was to cash in a bunch. The instructions with the checks said that if you're not using them in the US, don't try to use them as cash in retail or restaurant situations because it won't work. You carry them for safety until you need some cash, then you cash them in. Avoid cashing them in banks, try dept. stores.
Yes, you can safely carry more cash than you're used to. And Post Office ATMs will accept your ATM card and spit out yen and they are all over the place (once you know their logo, you can't avoid seeing them). It's best to think of Japan as a cash-based society except for major things like depatos and train tickets.