*pipes in as an unfortunate representative of the Federal Govt*

At work, we use U.S. and I believe that's the official deal because our letterhead reads U.S. Department of Labor, as opposed to American Department of Labor, or Dept of Labor, USA.

I don't think I've ever used USA except on international mail, cause like artbrooks said, the post office prefers it.

USofA is something I hear quite often though, living in Texas. But yeah, it's got a fairly backwater, 'god's country' kinda connotation to it.

Its often used as an education identifier. The most educated opting for US, the moderately educating choosing USA, and the uneducated employing USofA or America. (And recent immigrants often use America, from what I've seen.)

But mostly, the whole deal is up to personal taste.