Whenever I write a new article, I always use American English spelling.
When I spell-check an article, and British English spellings like "labour" and "programme" come up, I tell the spell-checker to calm down and accept these.
When I edit an article manually, I switch to British English spelling when I notice that the current paragraph is using British English spelling.
My motto is "blend in".
Ed Poor
wikipedia-l@lists.wikimedia.org