Little more digging, and I still can't find a whole lot on the subject. However, it does mention in OPNAV 1710.7A (Protocol Manual) that on the occasion of Formal, but not official visits that anyone entitled to a 19 gun salute or more should be afforded full honors. My guess is that you'd be safe saluting anyone that rates more than 19 guns, which includes:
President, Former President, Vice President, Governor of State, Speaker of the House, Chief Justice, Ambassador, Secretary of State, US Representative to the UN, Associate Justices of the Supreme Court, SecDef, Cabinet Officers, President Pro Temp of the Senate, US Senators, House Representatives, Deputy SecDef, Secretaries of the Army, Air Force, and Navy, Under Secretary of Defense; Foreign Dudes: President/Sovereign, Members of Royal Family, Prime Minister or othe cabinet officer.
They all rate 19 guns or more on departure. The only ones that rate them on arrival are:
President, SecDef, Deputy SecDef, Secretaries of the Army, Air Force, and Navy, Under SecDef; Foreign Dudes: President/Sovereign, Members of the Royal Family.
I'd say it's pretty safe to say that if you are ever faced with whether or not you should salute some of these people, you probably should. Or, you could be stationed at 8th & I/White House and already know if you should or not. In which case, may God have mercy on your soul.