Watch video for full explanation. Every new rotor is built to a run-out spec. Every existing hub on a vehicle (no matter how many miles) also has an allowable run-out spec. Exceed the spec for the vehicle and the car will very likely develop brake pedal pulsation over time caused by uneven wear. Just like tires need to be balanced to avoid vibration, so to rotors need to be matched to avoid vibration. In the case of the tire/wheel balance you will feel that immediately if it's off balance. With the rotor/hub match you will feel it 3k-7k down the road if it's off. The exact distance is impacted by many factors including brake pad composition, caliper style (fixed vs. floating), and condition and function of the rest of the braking system. Needless to say, not matching the rotor will cause a problem at some point. Matching the rotor, simply put, is eliminating wobble to below OE spec, so the car won't develop pulsation later. If customers do not wish to add the service on their new rotors, you can assure them that you have the solution should they need the repair in the future. Today it will be $X, since we already have the wheels off and car on the lift, but if you have to return later it will cost $X + to repeat the job.
