I think the only type of security sign that can actually be helpful is an alarm company sign. Announcing you have guns just encourages bad guys to be extra sure that no one's home when he comes to take your guns away from you.
I have to agree with this. In more aspects of life than just my home. I believe far too many people advertise who they are and what they do with stickers on the back of the're cars.
Unfortunatly this is extremely common in the military dispite being briefed and trained on keeping a low profile and not becoming a target. When my brother in law was deployed my sister had one of those magnet stickers that read "my husband is in afganistan" or something along those lines with US army wife writen under it. To me this just paints a target on her that her combat trained husband is no longer in the house to protect her.
Back on topic to home protection signs. I used to have those "protected by S&W" stickers on a couple of my windows but have since removed them. The same goes for luxury items, After 2 break in attempts my uncle removed his Porsche parking only signs from his garage.