In the English-speaking parts of fandom? Probably a good assumption, since there are an awful lot of American fans.

Still...if you go to, say, a Gay Pride rally, and someone says/does something stupid? It's probably not a great idea to say "They must be gay, then." Same applies to nationality, or any place on the internet that is predominantly one thing (female, American, black, gay) because people tend to think you're an idiot.