Do most dogs treat people based on their physical apparence?

In teaching kids how to be safe around dogs, they are told to ignore the dog,don't look directly at it, and stand still. The dog might bark because you're in it's space, or it doesn't know if you are friend or foe. Once it has a chance to come close and snif you, it can then clasify you, and will benerally then go off to find other things to explore. I think dogs in general tend to err on the side of caution and try to intimidate the newcomer in to leaving them alone (or maybe to come play?), and will continue to do so until they can check you out.