Better to be judged by 12 than carried by 6.
This is actually bad advice. An instructor for a tactical pistol training course I took a while back gave me a better rule: Justice In Three
You are in defense of yourself and your freedom three times when you use a firearm in self-defense:
1. When you fire your weapon: You are defending yourself from the assailant. It's best to shoot to kill, as the tale will be decidedly in your favor.
2. When you talk to the police: You are backing up your right to fire your weapon. If the perp's dead, tell the truth. If the perp's alive, request counsel before talking to the police.
3. When you tell your story to the judge/jury: If it gets to this point, you either have an overzealous cop or a live perp. Make sure you have a good lawyer, tell the truth, and make sure you explain that you had no options but to defend yourself.
These three decisions should be made before you even draw your weapon. They should consume your thoughts every day and every where you pull a weapon. If you're not prepared for every scenario, you're going to wind up killing an innocent bystandard (on your conscience), going to jail for manslaughter (on your freedom), or dead (on your life).