I think it pretty well depends on how wide the stripe is on your fabric -- I cut the stripe the way I want it to be, then cut my triangles. I must warn you about yardage of your fabric, though most of the time you can get at least three complete stripes. After all these words of "wisdom," I must admit I have not made the triangle frenzy pattern yet. I've taught the striped tablerunner, and feel the method would be much the same.
Linda in Missouri