US Campaign Ribbons

[MacMillan of Rathdown]

I don't know what "technically" you are referring to but I don't really believe the invitation has to specify decorations for them to be worn.

Actually, it does. US military regs specifically prohibit the wearing of uniform items (that includes ribbon bars and medals) with civilian clothing. The exception, in civilian attire, is when attending an event when it is specifically requested that attendees wear their military medals.

Invitations don't specify sporrans or other such parts of total dressage.

Actually, they do. "Black Tie" on an invitation means just that-- appropriate evening attire, and all that goes with it-- sporrans included.

My experience is that like other aspects of being kilted it is up to the person who is doing the wearing.

It is up to the person receiving the invitation to respect the dress code requested by their host or hostess, and to dress accordingly. Unless the invitation specifically states:

Black Tie, Decorations

or

White Tie, Decorations

then the wearing of medals is inappropriate, and would be perceived by most as a serious faux pas.