As a writer, the question for me is trying to make the characters as true to themselves as possible. Therefore, if that character is American, I try to make their speech and thoughts conform to American patterns; ditto for Brits and Canadians. And as a reader, I do find it distracting for a New York cop to say, "..., yeah?" at the end of a sentence or a USAF colonel to say "I've not been sleeping well lately," and if that pops me out of the story, even momentarily, it's a flaw in the story. Maybe not a huge flaw, but still.