I like spin balancing tires/ wheels on the car, mainly old spoke wheels cars with good results. In this case it would be a shame to rebalance tires to compensate for a poorly cast drum. Drums a d rotors are supposed to come balanced...kinda, every one I check off somewhat. Weld to the drum if needed though.
Did you notice a vibration or pedal pulsation after you put drums on? If not don't worry about it.