Hmm, but if I understand correctly how this all works, even if you vote and pay taxes in Smalltown, if Bigtown is the default placename for 12345, the post office will still happily deliver mail to your address even if the envelope says "Bigtown, EG 12345" on it.
Of course, humans care about this stuff more than you can imagine. There was a minor kerfuffle in the Buffalo area a few years ago when a woman bought a house in Lancaster (an upscale suburb) only to find that the default placename for her zip code was Depew (a working-class suburb). She somehow managed to get someone at the newspaper to write a story about her plight, which included a note that her copy of Gourmet Magazine came late because she insisted on putting "Lancaster" in the city name. I'm not sure why that would affect anything unless she was also deliberately giving them the wrong zip code though.