But when you're talking to a princess you would typically use the form Your Highness, or similar.

Anyway, I was thinking of the sorts of situations that actually turn up, on average, for most people. There are only so many princesses to go round, a tiara is highly unusual office garb, and I've never heard of anybody asking to be called "princess" in a professional capacity, at least not in white collar work such as Google. I stand by my rule of thumb. My difficulty thinking up situations was based on my experiences of being at work, which are of course unique, but not, I suspect, especially unusual.

(But if we're trying to think of unlikely possibilities, what about if Princess is your colleague's name? Though given your reply, I expect you're the sort of pedantic so-and-so who'd actually notice that I deliberately quoted it in lower case to sidestep this one.)