If you use Americanisms just to show you know them, people may find you a tad tiresome, so be discriminating. Many American words and expressions have passed into the language; others have vigour, particularly if used sparingly.
And thus begins The Economist’s style guide to using ‘Americanisms’.
Even though I’m not American, I am overcome with the desire to drawl, “Yeah, well we saved your butts in Dubya Dubya two.”
The full style guide is here.