I have called my teacher by his first name at times, and frequently I've called him Sensei. Usually, but not always, in the dojo, it's Sensei; in most other situations it's Jack, unless it's somehow Aikido related and it's important that we each recognize our relative positions in that context, or that others see him receive that respect.
The teachers I've been acquainted with who're from Japan are always Sensei, and I have no idea why, but there's such a gulf of experience between us, I'd simply not feel comfortable calling them anything else. I have friends who are way beyond me in rank, up to 6th dan, and if they've not been my teacher, except as a training partner, and often when they have, I still refer to them by their name and add "san" if they live in Japan.
Teachers I don't know well, but respect, will almost always be Sensei, whether they're American or Japanese.
I doubt rules are really as critical as a certain kind of awareness of custom and appropriateness.