But... my question is: when you say 2x10 is does not exceed 5:1, or 2x12 exceed 6:1, well that gets a bit fuzzy to me. As we all know, a 2x10 is not really a true 2" x 10" and so on.

So in reality, how would one apply that code?

§R802.8 Lateral support. Rafters and ceiling joists having a depth-to-thickness ratio exceeding 5 to 1 based on nominal dimensions shall be provided with lateral support at points of bearing to prevent rotation.