A good rule of thumb is to contact the police whenever you use a firearm to repel an aggressor. This includes the display of the weapon, either holstered or unholstered. This, of course, pre-supposes you are carrying the weapon in accordance with the law. It is amazing how many aggressors retreat and call the police. Being criminals, they naturally lie. So, if you are threatened to the point that you feel that you have to display your firearm, call the police. Look around at bystanders and ask "Did you see that guy try to attack me?" Go to anyone who says, "yes", and ask them to stay and tell the police what they saw. Again, you would be surprised how often someone will actually stay and tell the police what happened. This can save you a lot of problems down the road.
One more thing. Be reluctant to allow the police to impound your weapon. Some officers, especially rookies, will want to impound it as evidence, as you are essentially reporting a crime, assault. Being the VICTIM, not the suspect, make it clear that you expect to leave the scene with the firearm in your possession. Always request a supervisor respond to the scene. You well may be asked to surrender your weapon upon the arrival of police officers. This is not an unreasonable request. However, unless you discharged the weapon, they should need no more than the make, model and serial number of the weapon for the report.