Ignoring all the BS, Pres Reagan was the first photographed saluting his Marine guards. Others may have done so but there was no proof.
A salute is something ANYBODY can do to show respect for another. The hand salute used by the military is simply a US custom. Each nation has their own.
The point is that this shows the casualness of the president, a sign of how he feels about those military personnel around him who he clearly takes for granted.
Yep Brian Williams and NBC did a piece tonight that explained Reagan "Began the salute in the modern era." Hastening to add that Bush gave a salute while holding a dog, and Eisenhower never bothered, explaining he considered himself a civilian, and the salute unnecessary in his new capacity as CinC.