For instance, if I apply 2 shots of IS to my neck, and another 2 xs of ammo, are the effects any different than applying the shots of IS to my neck and the ammo to my wrists?
If you apply pheros to your neck, you will likely get more self-effects than if you applied them to an area farther away from your face.
I personally like to do both, usually one spray on my neck/chest area, and one on my wrists/forearms (and rub together).