GEO, it is my intention to make words rare if they're not common outside the US, just as I feel words should be rare if they're not common within the US. No doubt there are plenty of common words remaining that should be rare under that rule, but there have been quite a few American words re-graded to rare after queries were raised: coed, veep, narc, moxie, vise, realty, tony, etc.
Of course, tricky issues arise where a word that originated in one country has started to get some use in the rest of the English-speaking world. Each issue raised has to be looked at on its merits, but I tend to lean towards making a word rare if there is some doubt about it. That's because the disadvantage is not so great if a player finds that a word that is common to her is treated as rare in the puzzle, compared with the situation of a player confronted with a "common" word they've never heard of.
Of course, I'm always happy (and never shirty) to look into any words queried by forumites.