I suppose it's a question that can be transfered to any translated fandom, but it was anime that got me thinking :).
Personally even though I'm UK based I find myself using (well make that trying to use) US English to write the fics because usually if the anime shows are re-dubbed they are re-dubbed in the US, but I think it is possible to argue that Brit English would be fine too.
What do y'all think?
[Edit - When I say US or Brit English, I mean things like jumper/sweater, pavement/sidewalk rather than punctuation. Y'know the things that prevent mental images of big hulking great men in dresses when it's not intended ::g::.]