Putting signs up is not a good idea because it tells possible thieves what system you have so they know how to bypass it. If you don't have one then it's that much easier for them to break in. It also tells them that you can afford a security system and that you probably have stuff worth stealing.
That's one opinion, but FBI surveys of past criminals say that the majority of criminals will pass by a house with signs of a security system, as they're just looking for an easy score. Sure "Bypassing" security systems sounds legit if you've watched too many movies.