I have three kilts (one in process) that are pleated to the stripe. In two cases, it was in part to honor units that wear the tartans...

In the last case, the sett is very large, and I don't have enough material to pleat to the sett properly, so, I am doing it to stripe, and have plenty of material for a proper 8-yarder....

I don't think that there is any actual rule prohibiting anyone from pleating to the stripe any tartan...

IIRC, I read that military kilts are pleated to the stripe to reduce the amount of material needed (and it is easier to do)