Here's a rule of thumb that I carry over from putting lights on Xmas trees...

Look at the tartan and squint your eyes...

Is there a single strong color that seems to jump off the tartan at you? If yes, I generally can pleat it to that stripe. If not, them pleat to sett.

Fraser Hntg... the Red really jumps off against the dark brown and blue and green. I usually pleat it to that stripe.

The Stewart Hntg: The red AND yellow jump out, but the Yellow is brighter and jumps out more, so I generally pleat that one to Yellow if anything. I find that when we pleat it to the Red stripe, the yellow is overpowering and makes horizontal lines across the pleated section.

Ireland's National: Not much really jumps out by ITSELF on this tartan, so we normally pleat it to sett.