I have a fuzzy recollection, when I started first looking at Scene Indexes, that _IsObserved was used extensively in the _PrimsAdded, _PrimsRemoved, _PrimsDirtied methods of all(?) the Scene Indexes that shipped with USD.
Looking at the code now, however, I see several/many USD Scene Indexes that don’t do this check.
What’s considered “best practice” here?