I'm okay with POV changes that are clearly demarcated as such--alternating chapters for instance. But if a whole fic has been from POV A and then suddenly, we hear POV C's internal monologue--that gets on my nerves.

You are right that canonicity is a loaded term and I'm probably not using it here to accurately portray what I mean. I mean canonicity in that canon characterization, plot, dialogue, etc. informs your fic choices as an author and in those places where your fic departs from canon, you do so consciously and for effect. For instance, if you're writing Rodney McKay, you're aware that in canon he's often a belligerent asshat, even if you choose not to highlight that aspect of his character.