Sorry I am late coming to this thread, but my advice would be to pleat to the stripe, and here is why:

This tartan is a military tartan. Traditionally, military kilts were pleated to stripe. Not that a military tartan cannot be pleated to sett if it is a civilian kilt (not part of the uniform), but pleating to stripe would be a nod towards the tartan's military heritage.

Specifically, I'd pleat to the light blue/yellow stripes on the green, both because that is the obvious stripe, and also becuase tartans based on the Black Watch sett (which this one is) are typically pleated to whatever stripe is on the green portion of the sett, enabling you to work more easily with the large sett created by such tartans (as has already been discussed).

And lastly, I just think it would look sharp pleated to the line.

Of course, it is your kilt, and the best advice is to get it pleated however you want!

M