Dean's "right foot or left foot" problem with the Retreat (3/4 and 9/8) stems from the fact that this time signature has a unique and interesting feature in terms of marching. In all our other time signatures we are used to the idea that the accented beat in the bar falls invariably on the left foot. For example, in 2/4 and 6/8 we have a Strong first beat in the bar followed by a Weak second beat. The bass drum is therefore loud on the strong (left) foot and muted on the second (right) foot, and this continues for the entire tune.
In the Retreat things are quite different. Here we have a strong first beat followed by two weak beats, so the strong beat from the bass drum (every third beat) alternates throughout the tune between the left and the right foot - a feature unique to the Retreat.
Once this is understood we can cut to the chase regarding which foot should mark the very first note of the tune - and the answer is simply "It doesn't make any difference as long as everybody is together". That said, it should be noted that in a tune like "The Green Hills" there is a source of ambiguity because the first two notes (A and B) are intuitively seen as "outside the bar", although they are often not written that way, and the bass drum emphasis falls naturally on the following C. If the pipe major deems that the first two notes are outside the bar, then the tune will start on the right foot with the A.
As I have remarked previously, if you are not now totally confused then you haven't been paying attention.