Convertor equipped engines have O2 sensor's which maintain the fuel/air ratio at the appropriate point to allow the convertor to function properly. If anything, the convertor may not heat up enough to keep itself clean if idled excessively. Many ECM programs use an excessively rich wide open throttle mixture to heat the convertor and clean it. I don't know if Bosch does this, but probably. Idling should not harm a convertor if the engine is in good tune with a functioning O2 sensor.