Ethics and courtesies of branching into other tartans?

[Farmer Jones]

Whilst I chose to seek permission from the Clan Chief to wear Sinclair tartan as the name was lost through marriage a couple of generations ago, this was purely a personal quirk.
As a Scot, living in Scotland, I'd agree with views previously expressed. Please do enjoy wearing whatever tartan pleases you, with respect and delight.
The only exceptions I'd make, for myself, are specific tartans designed for specific groups such as Regimental or Corps where I had not served.

Bring Forrit the Tartan.

I did likewise write to the Clan Chief of McMillan almost 20 years ago to seek permission and acceptance. I maintained correspondence with him for a couple years as a result.