The Rulebook says: "After birds have been shot, all Guns shall remain quiet and shall not move their positions unless and until so instructed by the Judges. Judges may instruct Guns and their associated bird thrower to retire from the sight of the dog, or dogs, on line provided that every reasonable efforts must be made to insure that the movement of retired Guns is only permitted at a time and under conditions to minimize the chance that the running dog will be distracted by such movements. The permitted movement of retired Guns and their associated bird throwers should be limited to the minimum distance required for effective concealment. Such retirement should be in a direction away from the direction in which the bird is thrown."
So, the answer is, "It depends." Where is the retired bird in relation to the other birds thrown? Would the movement of the gunners be less distracting while another bird is thrown, or less distracting when the dog is sent for the "go bird", which isn't necessarily the same for every running dog, if the handler calls the dog off of the last bird down and sends for another bird.
Whatever is decided, I would choose to have them retire at the least distracting time for dogs picking up the birds in the "usual" order.
In any circumstance, it must be the same for every dog.
