You might consider asking for a zip code rather than a city. I personally live in an unincorporated area of Saint Louis county, and I was quite tempted to name one of the three surrounding suburbs (all of which are politically considered a city). While the decision to just put down "Saint Louis" came easily, I'm wondering how much internal debate I would have had had I lived in an unincorporated area in the middle of nowhere.

And then, of course, there's also the reason that just typing in a zip code is easier.