"Beware" signs may or may not contribute. (I've worked a lot of dog bite cases and I've never encountered a Beware Of Dog sign being a factor one way or another. They sell them at PetSmart, Home Depot, Target, etc. That's what people buy because that's what's available. Maybe somewhere they're considered a big deal, but I've never worked in any locale where an owner has been penalized for it if a bite occurred. But that's my experience. I can't speak for the entire USA, of course).
In many states, dog bite laws are strict liability unless certain very specific conditions are met. So if it's strict liability, it's strict liability (so, I guess, why NOT put up a sign?) The following are those states that have strict liability statutes, according to the Michigan State School of Law.
Provocations is often a condition, but if you're not home or outside when it occurs, you can't prove provocation.
Here is a state map so you can look up your own state: http://www.animallaw.info/articles/armpusstatedoglaws.htm
(I don't like dogbitelaw.com because the guy that owns it is an attorney who makes money off dog bite cases. I don't think he's particularly unbiased, so I don't trust his information.)
As Anne points out, local jurisdictions can modify state law somewhat.
My feeling is that if a sign keeps some moron off your property to begin with, well, that's a good thing. No Trespassing signs are excellent. You'll note that "Anywhere a person has a lawful right to be" is a phrase that shows up a lot in the strict liability laws (although these statutes aren't quoted verbatim). So make it clear that a stranger doesn't have the right to be in your back yard. Then you can certainly scream to high heaven that the police should be filing charges against the person who intruded, if something does occur.
I like Dog on Premises signs (PetSmart sells them) because they're neutral. Strangers technically do have a right to be in your front yard. The postal carrier, the UPS guy, the Fed Ex woman, even the annoying folks trying to sell new siding, Avon, or religion -- technically, your property has to be safe for access for people with a reasonable purpose for being there. And "reasonable" is usually defined pretty broadly by the courts. Just because you don't want new siding, doesn't mean they don't have a right to be there. They're providing a service just by offering to sell you their products. I know. It's crazy. But that's how courts have viewed it.
So my Dog On Premises sign tells people I have dogs (which are never in the front yard without me). And when they ring the doorbell, they hear loud barking. It's a pretty clear signal. The signs on my back fence (and the fact that those gates are locked) make it further obvious that folks aren't welcome back there.
But if I lived in Anne's jurisdiction, I'd have BAD DOG signs all around my property!