You can have a kilt pleated to any stripe in the pattern you want, for the most part. It is easiest, and usually looks better, if you select one of the "pivot lines" in the tartan. But technically you can pleat the kilt to any stripe you want. And what stripe you select can make all the difference in the world regarding the look of your kilt.
To illustrate this point, here are three different kilts made from the exact same tartan -- the standard Buchanan clan tartan in modern colors. Here is the front of one of the kilts, so you can see the overall tartan design (the front aprons of the kilt are not pleated).
As you can see in the picture, there are three main stripes you can pleat to. You can pleat to the white stripe, on the red background (showing mostly red in the back); you can pleat to the black stripe on the yellow background (showing mostly yellow in the back); or you can pleat to the blue stripe on the green background (showing mostly green in the back). As I am making box pleated kilts, the size of each of the pleats will be wider than in most 8 yard, knife pleated kilts, and show more of the background color. Here is this same kilt shown from the back.
On this kilt, I pleated to the white stripe on the red. This would be my personal preference for pleating this particular tartan, because I think it gives the most balance. Out of the three dominant colors (yellow, red and green), red is the "middle ground." It is less bright than the yellow, but brighter than the green. This does not mean that the same tartan would not look striking pleated to one of the other options.
Here is a different kilt made from the same tartan (it is a larger size and so has more pleats). This time I pleated to the black line, which shows mostly yellow in the back. It is a much brighter kilt than the previous one, pleated to show the red. You can see how this has created a very different look. But there is still another option for pleating this tartan.
For this last kilt, I pleated to the blue line, showing mostly green in the back. This is a very different look, because by pleating to the darkest color, the brighter red and yellow lines in the sett really show in the horizonatal. It is visually a very different kilt from the other two, though it is made from the exact same cloth.
So, when ordering your kilt, in addition to what tartan you select, also give some thought as to how you want that tartan pleated. The stripe you pleat to (assuming you have it pleated to the stripe) can greatly affect the overall look of the garment, as I trust these photos have illustrated.