Is recharging a magneto rotor magnet necessary?

Not during routine maintenance such as a 500 hour inspection unless the rotor or coil are removed from the magneto.

I see this as two separate issues:

1. Assure that magneto meets tolerance and specification compliance,
2. Maximize magneto performance

1. The allowable tolerance might be robust enough to allow the magnet to remain within specification in the anticipated operational and service environment. Minimum coming-in speed^1. can assure that the magneto as a whole is within tolerance and indirectly that the rotor magnetism is within tolerance. In this context it might be unnecessary to re-charge rotors.

Many magnetos lack a documented service history so it is possible that the magneto rotor at some time could have been exposed to unanticipated and/or unusual situations that are outside of the manufacturer's definition of the service environment. One example might be fire or someone else attempting to re-charge the rotor with improper equipment or technique. In this context, replacing, re-charging, and other non-typical inspections are necessary to restore the rotor.

2. Proper recharging and installation can assure the magneto produces as much current as it is capable of. This can be desirable. Peak performance, rather than a range of performance within a tolerance zone might be a safety issue. Peak performance might allow the magneto fire the plug over a wider range of E-gaps than a magneto that is still somewhere within the tolerance zone but not at the top end.  It might also help starting performance by producing more current across the spark plug gap.

There could be a conflict between these two issues as peak performance might fall outside the tolerance zone for some reason. i.e. manufacturer wishes to "de-rate" the magneto. This would imply a lower bound to the coming-in speed tolerance.

Removing the Coil or rotor:

Exposing the rotor to a open magnetic circuit for the first time can drop the magnetism from E to F.

Magnetic Hysteresis Curve

1. Minimum Coming-In Speed is the minimum speed at which the magneto will spark a 5 mm gap of standard design.