I love it — I love the word obsession! Design details such as this really lend to an application’s overall design “hygiene.” And like you said, consistency is key, whatever is chosen.
Target is another example that uses case very well; the functional UI portion of their site (navigation labels, cart & checkout labeling) are lower case, while the products they sell are title case, which puts a subtle emphasis towards sales.
It’s also interesting to note that you didn’t once reference the AP Styleguide or the Chicago Manual of Style…dinosaurs of authority, perhaps?? ;-)