"Rolling" is a relative term. Some applicants with packets complete early, will hear early. Those tend to be LOA recipients with Pres noms and then principal nominees with or w/o LOAs.
Once MOCs start submitting their slates, additional appointments start going out. Initially, those are likely to be LOA recipients who now have received an MOC nom. It could also be applicants in areas where very few qualified candidates received a nom -- typically b/c the MOC received few applicants -- in these cases, it may be relatively easy for USNA to select the slate "winner."
The majority of appointments are sent out in Feb. and March and these are made up of those winning their slates and from the national pool. Even these are "rolling" in that there is no one day on which everyone is notified or even most people are notified. USNA commits to notifying everyone by April 15 but, in fact, the overwhelming majority of applicants will learn of their decision prior to that date.