We waited until we were sure. Once sure, we told the schools quickly. At this point (mid January) a few years ago we were still definitely undecided.
One daughter attended a school where you get to be assigned to a dorm as soon as you tell them you are coming and pay your deposit, which was a reason to decide a few weeks before the deadline.
I am under the impression that schools do not overbook generous merit scholarships as much as they overbook on acceptances. For this reason we did decline a “super safety” that had offered a very generous merit scholarship (nearly full pay) as soon as we knew D2 was going elsewhere, even though she hadn’t decided yet between 3 different schools.