Unlike the others, I would wait until after the interview - that way you have a card left in your hand in the event you get waitlisted. If you get accepted post-interview, then it didn't matter. If you get waitlisted, you can play that card later in the form of an update letter.
I highly doubt that, in the big scheme of things, this past semester of grades is going to influence a reject vs. waitlist vs. accept decision at this point. An update letter in the future, however, very well might be the difference between an accept vs. reject decision if you find yourself on the waitlist. An update on grades is a great reason to send a letter if necessary and, in so doing, you can also squeeze in the other "juicy" stuff: interest in the school, updates on ECs, etc..