There was a thread about pleating to the line on another forum a while back, and a kiltmaker chimed in and said that certain tartans aren't all that suited to it, because they don't have a line that's flanked by wide enough bands.

Her point was that if the pleats had enough taper, and the bands of colour which flanked the line weren't wide enough, additional edges or lines would appear partway along the pleats, an effect she said kiltmakers want to avoid. Now, she's talking traditional 8-yard kilts with rather narrow pleats, perhaps only 1/2" wide each or so.

So for this particular tartan she would, I suppose, want to pleat to the white line as it's flanked by wide green bands and the pleats could taper without anything other than that green showing. If you pleated to the yellow line, and if the pleats were wide enough and tapered enough, red lines would appear partway along the pleats.

To me, pleating to the line is nearly always preferable for the simple reason that all kilts were originally pleated that way, or pleated randomly. Pleating to attempt to simulate the unpleated portion is a relatively recent thing.

And, when a kilt is pleated to the line you get two looks for the price of one because the front and back of the kilt look so different.

There are some few kilts I've seen that the wrong portion of the sett was chosen for repetition so that the back of the kilt looks much less attractive than the tartan itself.

I guess some tartans' beauty depends on all the colours being seen together and no matter what portion is chosen for repetition in the pleats, the beauty is diminished.