This is a myth, no idea where it came from, but dogs will sniff you if they want to come sniff you. Body language is important and it’s important to give the dog agency in the greeting. Putting your hand out towards a dog is not good advice for most people as most people don’t understand body language, can be threatening to the dog, and it’ll probably get someone bit by a nervous dog.
In general, just ask the owner for permission and information, or keep a comfortable distance and leave the dog be. Dogs are still animals, even if they’re usually friendly.