As for the terms "rationalism" and "rationalist," they have a strong established historical meaning quite different from the way they're commonly used by many people here. The first thing that occurs to me when I hear them is the old philosophical notion of "rationalism" as opposed to "empiricism."

Also, it's important to note that historically these have never been terms of uncontroversial and unambiguous praise. In many contexts, they have been traditionally understood to convey criticism, not compliment. For example, when Michael Oakeshott titled his essay Rationalism in Politics, he definitely didn't aim to make the reader positively disposed towards the subject from the title. Whether and to what extent people on LW tend to commit the same errors and hold the same unsubstantiated beliefs that have traditionally been connoted by this term is certainly a complex and amusing question.

As the bottom line, the "rationalism" terminology is probably too deeply embedded in the LW folklore to ever be abandoned, but I would certainly advise against using it when talking to outsiders. Even if people understand the term precisely the way it's used here, describing oneself like that explicitly is a status-lowering way of qualifying oneself.