Never.
Do you own a friearm and are proficient with that firearm at present?
NO........then carry a rock
YES....then you know that the only reason to pull a firearm is to shoot/kill someone otherwise carry a rock.
X2. If you are not ready, willing and able to carry said firearm please leave it home. If you are not PROFICIENT to the highest degree with said firearm please leave it home. Make sure you have a lawyer on retainer because if you use said firearm in self-defense and/or are not permitted properly you are going to jail first and questions will be asked later. If you cannot meet the above requirements and still wish to carry please be prepared to accept the consequences. "Better twelve as my jurors than six as my pall bears" only goes so far if you injure/kill an innocent bystander.
FYI I am a life member of the NRA, am a certified firearms instructor in five areas and a certified small bore jr. rifle coach. I am not telling you what to do, I am merely requesting you take ownership and responsibility for your actions and their potential consequences.