I would not and do not proclaim that we have firearms. I agree with the posters above that it says you have something worth stealing.
We do have a monitored alarm system. I do not have their signs up. I would rather use a generic warning sign. I figure it provides the same level of discouragement without giving the experienced burglar the knowledge of which system he is attempting to get through.
I like the beware of dog sign because neither of our dogs is a danger to anyone. They do make a lot of noise however. That should be enough.