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.
Examples of such tags: lusaka, kampala, bavaria, parnu, ko-phi-phi
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.