I found several questions, that mention a specific city and also are tagged with that city name. For big, popular cities like london that makes perfect sense, but in most cases they will get few question. I would stick only to country tag.
All of them have currently only one question tagged and are not very likely to get many more soon.
There has been discussion before on single-use tags. Tag cleanup - single-use tags. I propose a rule to avoid city or region tags, that are likely to become single-use.