Took a Vehicle and Street Tactics class. Two scenarios and what they suggest:
You find an armed BG
In your command voice, with your firearm in low ready, you shout out, "Police....drop the weapon".
You are in a store/restaurant/public place and shots are fired. You draw your firearm.
Whether you shoot or not, right now, you look like the bad guy. You should be repeating the phrases:
"Police, somebody call the police"
"Police, is anyone hurt? Somebody call the police"
"Police, don't go near him, he still may be dangerous" (assuming you shot the BG and he is bleeding on the ground)
The theory behind this logic is that in the first scenario, you are NOT saying you are the police, but if the bad guy is actually a good guy, he will not try and shoot you as bad guys do not ever say the word police. If he is a bad guy, he may be more inclined to comply.
In the second scenario, again you are NOT saying you are the police, buy you want everyone to know that you are not the bad guy. Bad guys do not ever say the word police. If a crowd sees a person with a gun and there was a shooting, they will assume you were the shooter. This prevents other CCW holders from actually trying to shoot you, too.
Another point the instructor made was that you are in no danger of "impersonating" a police officer. People do it all the time, for example at Halloween and such. The only time you can get in trouble impersonating a police officer is when you do it for personal gain.
Just a few thoughts.