You could put one up high to verify that the flow across the sail is from leech to luff when running deep. I have one of those little flags at the top of my mast for that.
I think the Rooster article said it pretty well. They need to be about 1/3 of the way back where the the sail is deepest, and someplace where you can actually see them. I like to put them at to or three heights so I can see how the air flowing across the whole sail, though I have to deliberately look up to see them.
Putting some along the leach of the sail may help when sailng by the lee downwind.