Reminds me of a rule I heard for writing stories as a whole, which is that the central character should be the one with the most at stake in the story's events. Which, again, doesn't work for a lot of mysteries or client-of-the-week series, where the main characters tend to be detectives or lawyers or doctors or fugitives who are helping out the people who have the most at stake. Which is maybe why these days we have so many client-of-the-week TV series where the clients' cases always have a coincidental resonance with whatever personal crises the heroes are dealing with at the time.
Yeah, the one time Terry's Rule failed me was when I was writing CSI books, since those were more about the forensics than any personal dramas or conflicts. Who should be the POV character in a scene that's all about finding fiber evidence or comparing mitochondrial DNA? In those books, I just tried to balance things out among the regulars so that nobody got neglected.
"Hmm. Been awhile since we've done a Catherine scene. I guess I better have her find the bloodstain . . . ."