There is some subjectivity, certainly, but familiarity with Imperial busts and standing
portraits does suggest that your first one is right, that is, what a
Roman would not find anything noticeable about.
My own practice is analogous to that for a standing figure, where the 7th cervical lines up with the heel of the supporting
leg (we usually do have at least subtle contrapposto, sometimes more, following Greek precedents).
That is, I like to imagine a plumb line from the top of the cranium through to the center, as it were, of the neck; that LOOKS right-side-up and comfortable (based on
Roman art) to me.
Note that the designers of modern
portraits on postage stamps, such as those of Elizabeth II, are that way.
Pat L.