You may be forgetting cast off in the stock.With a rifle or shotgun that has a single barrel or an over/under you are correct as the line of recoil is along the centerline of the gun so shooting position has little effect. Here I would agree that whatever works best for the shooter is the way to go. With side by sides the line of recoil is either left or right of center line of the gun. There is a reason barrels of side by sides converge towards the muzzle.
Even single barrels and superimposed shotguns have cast off. This cast off will induce some sideways motion to the recoil impulse.
It's all a mute point really. The recovery from any chosen method of mounting in time is milliseconds and certainly quicker than cycling a bolt.