I cleared this up, together with a number of other ESTA related questions, during a long connection at IAH. The office dealing with global entry was empty, and the agents sufficiently bored to put up with my questions .

But ESTA enrolment isn't that taxing, will need to be sooner or later, and isn't exactly expensive. So, if doing it earlier than required brings comfort, however irrational your concerns, then I'd get it over with.