AT is correct on the importance of not posting a sign that would come back to haunt the owner in event of a self-defense shooting. Some of the signs are humorous, such as the one announcing that "survivors will be shot again," but they would be fodder for the prosecution or plaintiff in after-event litigation -- criminal or civil.
I personally have not posted either our home or our weekend cabin with signs indicating the presence of firearms. As I see it, such signs might encourage a home invasion by some "BGs" looking for guns whenever the premises appeared to be uninhabited. Signs warning about alarm systems, however, are a different matter.
It goes without saying that a high-end, fire-rated gun safe internally bolted to the floor of each residence provides great piece of mind.