@John Moore Williams: Yep, totally agree with you there! I intentionally simplified my definition of title case because different style guides follow different rules for title case.

When I was researching this, I did come across a small number of cases where products used start case—a variation of title case where every word is capitalized, even prepositions. Here’s one example where they use start case for buttons and nav items: http://gengo.github.io/style-guide/voice.html#style-guide