I think stripe pleating works beautifully but not in the case of Black Watch. effective stripe pleating needs great contrast in the vertical stripe to overcome the inevitable horizontal banding effect on the the back of the kilt. As you know BW a very dark tartan and doesn't have this contrast so I think to sett would be best.
Have a look at this picture above. It's a 13oz wool, pleated to sett and shows good contrast between the vertical and horizontal when pleated this way. The vertical stripes are wider in width and more widely spaced but show the contrast well. If you were to pleat this to stripe I would think the best option for contrast would be to go for the dark thin vertical in the lighter green. Now look at the picture again and imagine it pleated that way and I think it would emphasise the Green/Black/Blue horizontals too much, effectively removing the vertical stripe from view.
Compare with this picture of a Johnstone 13oz wool where it is pleated to gold stripe. The banding effect, of the darker horizontals, although visable, has been minimised by the contrasting verticals working with the three gold horizontals so emphasising the vertical stripes, which is the whole point.
Just my opinion but I think, with a tartan that does not have much contrast? Go for sett pleating.