I said this in a predecessor to this thread a long time ago, but I don't know of any cases where DCC is advantageous now. (There are price shocks and such, but these are the exception and I've been finding that most of my cards now lock the exchange rate to the transaction date, not the posting date.)

Historically, US cards had a currency exchange fee, so there was no additional fee from the card issuer if the transaction were in USD. Now, cards seem to all have a foreign transaction fee (which could be 0%). That switch removed any potential incentive for DCC. Furthermore, DCC used to be lower, around 2.5-3%. If you had a currency exchange fee of 3% and the DCC fee was no more than 3%, it would make sense to select DCC. You'd also get rewards on the full transaction amount that includes the DCC conversion rate whereas a currency exchange fee didn't earn rewards.