I would say go with whatever works for you. Try shooting both ways and see which one gives you better scores. I don't see the harm in using rifle technique when you are AIMING a shotgun at stationary targets on the ground, just as you would a rifle. I could, however, see how this would be a problem when someone, used to using rifle technique, switches over to shooting clays/birds and defaults to the learned muscle memory rifle form and can't hit anything moving/flying. But it doesn't sound like that's a problem for you as you've already established good wingshooting form, but could be something to watch for when your student gets to that point.

Mike