I agree that this is a good idea. This may be particularly important for students who have been accepted to graduate programs, and for students who got very large merit scholarships.
Undergraduate programs will over-book to some extent, assuming that some students will not show up. I do not think that they do this for doctorate programs (whether PhD or some other doctorate). There is also probably a limited extent to which they can do this when a large merit scholarship is involved.
One daughter was waitlisted at a DVM program. She was accepted off the waitlist before May 1, implying that at least for this particular DVM program they were accepting people off the waitlist before the May 1 deadline.
Our other daughter, years ago, applied to one university that was probably too much of a safety and was offered nearly a full merit scholarship. We had her let them know as soon as she was sure that she was not going there. Our thought is that someone else would most likely be offered the money that we were turning down, and our safety was someone else’s dream school.