There's an even simpler rule: The more convenient currency conversion is, the more you are getting ripped off.
That simply isn't true in my experience. For years, I used a debit card from Nationwide in the UK to withdraw currency all over the world. They used the VISA wholesale rate with no vig added on top and no fees for ATM use. In total, I withdrew probably over ten thousand dollars on that card, and it was one of the most convenient currency conversion systems I have ever used. (The card now carries fees for the service).
On the other hand, going to a high street bureau de change would have been much, much less convenient and would have secured me a worse rate in every case. Traveler's cheques would have been both inordinately expensive and hideously inconvenient. In fact, in the absence of a local bank account in each country I visited, I'm still not aware of cheaper way to convert currency than the system I used.
If you travel a lot and can find a bank or credit union that won't charge you any fees, hidden or otherwise, you get great convenience and an excellent price.
Yep, you and thousands of others. Unfortunately expats ruined it for everyone else, simply using it as a way to draw money from their UK accounts when retired in Spain or wherever, never giving a penny to Nationwide. They probably made a loss on each of these customers so they withdrew the service.
They really just charged the Visa wholesale rate for ATM withdrawals and purchases abroad with no fees or markup or anything. It was fantastic.
If I withdraw money from a Barclays ATM in London (or Deutsche Bank in Berlin, etc) using my BofA ATM card, I don't seem to have an extra charges.