never had a problem with my Wells Fargo ATM/debit card at a post office.

However, keep in mind you're going to get hit for several fees on each transaction. Not just for out of network but for currency conversion; and of course there's no telling if you get a great conversion rate or not. But it is darned convenient to get yen out of a machine when needed.

FWIW, my bro in law had no trouble with his cards in China but could only use the PO in Japan.