State laws vary. In Illinois, Beware of Dog signs are the kiss of death. You are admitting that you have a vicious dog and are completely open to lawsuits, even if someone comes into your fenced in yard without permission. The No Tresspassing signs or Tresspass at your own Risk Signs are considered preferable here.
Even if someone breaks into your house and your dog bites the thief in the act of robbing your home, unless you can prove that you were in mortal danger (held at gunpoint) the theif can sue you and prevail. There was a case not too long ago that the thief was shot by the homeowner, sued and won damages for loss of income!
Once again the legal system is more protective of the criminal than the victims.
Stella