I've heard - and I'm not a lawyer so take this with a giant grain of salt - that you can actually be admitting liability if you use "beware of dog." It's like you're tacitly acknowledging the dog is a hazard. If your sign says "dog on premises" it's supposed to be better.
See I saw the opposite posted by a lawyer in MI (my state). He was saying that if you DO have a dog that might bite someone and/or is protection trained, then you're better off having a warning sign.
The police in my city also advice people to have these signs even if they don't have a dog, it cuts down on burglary.
Bottom line is, anyone can sue anyone else for anything and if your dog bites someone, you are probably liable to some extent even on your own property.
I don't really like the BEWARE OF DOG signs, but I do have a sign on my gate. It is white with black letters that say "Keep gate closed, dog loose in yard." It doesn't imply that my dogs are aggressive or dangerous (maybe they are, maybe they aren't...) but does imply you may meet a loose dog if you open the gate.
We had issues with neighborhood kids reaching into our yard at our old house. There were huge backyards and no one had a fence until we put a 4' one up well inside of the property lines, but kids still came into our yard and we were always asking them not to reach over the fence. We also had once instance where an adult neighbor was reaching over the fence and waving a toy around because he wanted Nikon to jump around and bite at it and he would jerk it away at the last second, and he got accidentally bit because he misjudged how quick my dogs are, but he laughed it off because we'd already TOLD him please do not reach in or try to play with my GSDs. So at our current house the fence is 6' high, included the chain link double gate across the driveway. A professionally installed, well kept 6' fence and gate has served us a lot better. If a kid were to actually come up the driveway to the gate and reach in through the wood panels or chain link, well, I can't prevent that. Some of my dogs are not OK with strangers walking up or entering the yard, but none of my dogs are the nippy kind that would bite at someone's fingers through the fence, they'd just stand at the gate and bark like crazy. The mail lady has my permission to reach in and give them treats, but my dogs aren't left outside if I'm not around.