From what I’ve read, 1CosmicPossum is right- it’s not required or even expected, it was something Reagan started becuase supposedly he felt strange not returning the salute because of his service history. Like I said, the actual rule is that no one in civilian clothes is supposed to return a salute, though I know…

Actually, as a civilian, the Commander in Chief is not required to salute troops. No president before Reagan did so, even former military men such as Eisenhower. This tradition began with the master showman, Reagan.