When it sets your default for you, would that apply to posts or comments you email in, too? Or would you still have to identify if you want markdown at the top of your email?

I think editing a post to a different format should also set that format as your default, if we’re going with using a format sets it as default. I think if anything there should be a ticky (possibly on an interstitial page) asking to check if the user doesn’t want it set as the new default.

Otherwise just don’t set it automatically at all anywhere, and always include a ticky for making it your default. Those are the two options I see. I don’t see a compelling reason for the editing page to behave differently than post or comment.