Not OP, not from the US and never been in Japan but I travel a lot in general: always assume local currency, at the very least it's going to be cheaper and quite probably only very touristy places or airport only might accept $$. Here you can pre-order cash at your local bank and return any excess notes after. Exchange booths are usually a rip-off, and withdrawal abroad might cost extra, check with your bank.

Also don't assume people speak English. They might, but treat that as a bonus.

Local transactions would be in Yen (also all vending machines such as train tickets - very important for getting around).

I would say you'd want to have at least a few hundred dollars worth of Yen when you arrive, and you could possibly get more when you get there, depending on how your bank works. Another option is to get a prepaid Visa card which you can then use to withdraw money in Yen from their ATMs.