Ah so. The angels chorus and I have some better idea of why jaws should be cut under load.
I just watched this video to get a better appreciation of how a hydraulic chuck worked.
It appears that under no part load, the jaws mounts don't bottom out on anything when the draw tube plate pulls up against the inside of the back mounting plate. Besides a bit of sliding play there's also a little bit of lateral play in each jaw block track.
It seems to me that using a jaw boring ring gives the jaws something to bear against to eat up this play during the boring. I wonder if I should be pushing the jaws open against the boring ring when setting up for OD clamping. I'm thinking that a tip load on the jaws from the boring ring would result in a little runout in the jaw surface so it'd be a bit conical. Boring with the jaws pushing open would put the smallest diameter at the ends of the jaws instead of the root so whatever small amount of play would give more contact from the tip to the root than if the jaws were being closed against the ring which would make the diameter at the tips larger.