Three CC transactions in Spain so far. All three got the DCC prompt but none of them asked me about it. Two charged USD, one was nice and charged EUR. One of the USD shops was a trashy souvenir joint but the other was a pretty friendly and helpful hostel. Honestly, I think there's some confusion about DCC on the side of the merchant as well, so it probably pays not to be belligerent.

How do I say "please charge me in Euros" in Spanish?