"Mazda recommends rebalancing the entire rotating assembly if you mix and match rotors."

Remember that the counterweights - front and back - are there to offset the weight of the rotors. If you change the rotors, you are supposed to use the counter weight and counterweighted flywheel to match them.
So, if you seek the the lightest stock rotating components, you use the '84-'85 rotors, with the '81 flywheel and counterweight. (Strangely, that's legal in SOWDIV Spec7, but not in ITA). Because the counterweights are sized for the heavier '81 rotors, balancing is required.
BTW, even a stock rotary benefits from balancing. You can feel the difference, and all that unneccessary motion from a slight imbalance has to cost something.