But how did it compare to the rate in the currency app or xe.com for that day? Presumably you didn't make the reservation that day.

I think it's always better to decline DCC.

As far as the ATM, the ATM doesn't do any conversion as long as you opt out of DCC. The debit is done in local currency and converted by the network/US bank. Once you get home you can calculate the rates from your statement using calculators and historical exchange data on xe.com.

-David