A possible dancing myth?

[MacMillan of Rathdown]

Now that Diane and I are married she's made it quite obvious it's time she had a sash in a Donald tartan.

[I]The correct method of wearing Ladies Sashes

These three versions have the approval of Sir Thomas Innes of Learney, one time Lord Lyon King of Arms. They also appear in Frank Adams book, "The Clans Septs and Regiments of the Scottish Highlands", and in "Tartans and Highland Dress" by MacKinnon of Dunakin.

1. The sash is worn over the right shoulder, across the breast and secured by a brooch on the right shoulder. This style is worn by clanswomen, women of the clan surname and septs by birth or marriage.
2. The sash is worn over the left shoulder, across the right breast and secured with a brooch on the left shoulder. This style is worn by Lady Chiefs, chieftainesses, the wives of clan chiefs and chieftains, and the wives of colonels of Highland regiments.
3. The sash is worn over the right shoulder and tied in a bow, or secured with a pin, at the waist on the left. This style is worn by ladies who have married out of their clan but who still wish to use their original clan tartan.

Like you said in the quote above "The correct method of wearing Ladies Sashes."

The answer is #1. Even though she is a member of the RSCDS, she would only wear her sash on the left shoulder when participating in one of their events. The rest of the time she should respect social convention.

You mentioned Frank Adams "Clans Septs and Regiments of the Scottish Highlands"; you may not be aware of the fact that the book was considered seriously flawed and was revised by Sir Thomas Innes of Learney while he was the Lord Lyon, King of Arms. Sir Tam was careful to record many of the social customs (including ladies wearing sashes) as a guide for those who didn't know the rules so they wouldn't be embarrassed in public.