IMO the concept of incline for upper pecs came from concept of gravitational pull(90 degree to the floor) and the muscle that are aligned directly to the effective line of pull or path of motion, that is if you're pressing the weight straight up like smith press. inclines done with wide grip and bar directly in line with the neck or collar bone does seem to emphasize upper pec a lot more(like that Jason Huh video).
take a look at 5 different people doing inclines in the gym, chances are they're all doing it different somewhat. those who mastered posing(MMC) like high level bodybuilders will have best chance of doing it right, or optimum I'd say.