I have only encountered one merchant asking which currency I'd prefer, and that one is 100% consistent in doing so. And never bats an eyelash when I say baht.

Also only ever had one instance of having a transaction automatically processed in USD without having been asked our preference (beware of Villa markets). The clerk was very non-LOS about it when we insisted on having it voided and reprocessed, especially when the system crashed right after refunding the USD transaction, forcing an old-fashioned manual plate process for the baht transaction