Another factor that doesn't strictly come into play in formal wear rules, but is none the less an issue with highland dress is the overall Scottishness of the event. A good example would be Dirks.

I own two dirks, one with a (fake) Jeweled top and one with plain metal. While I have been told originally that they were day and evening versions, I would not wear either at an event that was not Scottish in nature. What is acceptable at a Burns dinner, in a room full of other peacocks would not likely be acceptable at something where I was the only kilted attendee regardless of formality. At most black tie events, no one else is wearing open weaponry.