When writing in Chicago style, you do NOT use periods for any abbreviation that is all caps (NATO, US, OPEC, etc.) even if lowercase letters appear within the abbreviation (PhD, for example). The exception, and of course there is one, is that if you are writing for a publication that uses traditional state abbreviations with punctuation (N.Y. instead of NY, Ill. Instead of IL) then you would also use punctuation in U.S. Chicago recommends using the newer two-letter non-punctuated postal codes whenever possible, though, so you would also type US w/out punctuation.
The MLA style also follows this pattern. I suspect it is the norm but suggest that you doublecheck the styleguide for the style your publication follows to be sure.
