If anyone's wondering, the one instance of DCC I encountered in Japan actually went as well as DCC could go. I'll upload receipts later, but the gist of it is:

1. Foreign card inserted, terminal asks if merchant would like to perform DCC.

2. Terminal prints out a "Service information" slip. It clearly lists the JPY amount, home currency amount, the exchange rate applied, and says to push OK on the PIN pad to accept or Cancel to decline. It very clearly says "transaction will be based on Reuters Wholesale Interbank exchange rate and includes a 3.50% margin".

3. If you decline, a DCC-free slip prints out. If you accept, you get a slip that shows the exchange rate, transaction currency, and transaction amount in home currency.

I was using my CAD credit card at the time and basically did it to see how it worked.