How is an initialism used and what form it portrays is a matter of how proper the formal paper is supposed to be. However, even if initialisms are acceptable, it is worth noting that some style guides assess the acceptance of U.S. as a noun and U.S. as an adjective differently. Have they applied these techniques to this area?

What is the advice of The Chicago Manual of Style (2003): 15.

What is U.S. 142 as used here? In short, S. Or US. ” Aside from scientific practice, U.S. traditionally appears with cycles. In most contexts periods and phrases are always omitted. What is best done by tradition over consistency so editors can shine themselves? In running text, the abbreviation (in either form) is permissible when used as an adjective but United States as a noun should be spelled out.

This guideline is, of course, only one style manual’s opinion, but Chicago carries a lot of weight in U.S. publishing circles.