I think it's a great idea not to advertise at all, no matter what caliber you choose to put on the sign.
Besides the reasons above - some criminals just don't give a damn. What's to stop some thug from casing your house one night, seeing the sign, and then coming back later, armed to the teeth himself? Nothing. Of course, you might have that happen anyway. My point is simply that the less the criminal knows about the contents of my house, the better. I don't want him to break in at all, but if he does, I want him to be anticipating as little resistance as possible.
That way, I (or anyone else caught in the house during a break-in) could very well be allowed a few extra seconds of reaction time when the badguy is confronted with a weapon, and caught off guard. If I've got signs posted all over warning of the wicked bad firepower I've got waiting for them, that takes away that little bit of advantage, for absolutely no measureable gain.