I wouldn't advertise. You leave yourself open to car jackings, home invasions, break-ins, etc.
So you don't leave your gun in the car/truck -- fine. But someone just has to break into your glove box, find your home address on your insurance card/registration, etc. then scope your house out for when you're not home. Bust in with the proper tools to take down safes, lock boxes, etc.
Or, wait for you to return to your vehicle from the bar/store/etc. knowing you are likely armed, and get the drop on you.
Forewarned is forearmed, as they say, and I choose not to advertise the fact that I have at my disposal, and may use, deadly force.