Do you think you may have an unacceptable leak in the system? If so, you may want to add some leak check dye to verify before you use up your precious R12. As said above some slight leakage over time is OK. Adding oil or discharging the system would be a last resort only if you're sure there is a problem warranting it. If you can find someone that can service with "Freeze 12" (totally compatible with R12), that would also be another option, IMO.