I just got back to the US from the past two months in Japan.
If you are staying in Tokyo... then you'll find an awful lot of places take credit cards. If you go out into the boonies..... it is a cash society.
Traveler's checks are almost impossible to cash outside of big cities.
The only thing slower than dealing with Japanese banks is ....... hum.... I don't know what is.
Try to not need to find the answer to this question.
Using 10, 000 円 bills is no problem whatsoever ....even for a stick of gum in a Lawsons (which ARE everywhere).
best,
...............john