Gprims are boundable, and should always submit to computing an extent (even if it be an empty extent), and valid UsdGeom scene description should always provide an authored extent on a gprim that captures its changing shape (if its shape is animated).
We therefore should calculate a prim's bounding box and call SetExtentsHint.
The USD glossary says:
We therefore should calculate a prim's bounding box and call SetExtentsHint.