John, I just did some Googling and found the hand salute described variously as a courtesy, a custom and a requirement.

However,I did find, under Offenses Punishable by the UCMJ, an interesting reference to UCMJ Article 89 at . . .

http://www.recruitknowledge.com/pages/gms/ucmj.htm

. . . which states,

"b. Article 89, Disrespect Toward a Superior Commissioned Officer: Any behavior which is disrespectful to a superior commissioned officer may be punished by this article. Failure to salute can be interpreted as disrespect towards a commissioned officer."

That would indicate to me that a hand salute, rendered or not to a commissioned officer is taken to be more serious than is implied by either a "custom" or a "courtesy?"

When I was in uniform, this is how I interpreted the hand salute, although at the time, I was not aware of UCMJ Article 89.

At http://www.ucmj.us/, there appears the following,

889. ARTICLE 89 DISRESPECT TOWARD SUPERIOR COMMISSIONED OFFICER

Any person subject to this chapter who behaves with disrespect toward his superior commissioned officer shall be punished as a court-martial may direct.

I would interpret this to imply that failing to render a hand salute must have found its way into Article 89 by way of UCMJ Case Law since it does not appear verbatim.