First, there’s no rule that says your dog has to meet every dog they come across. But, they should learn to maintain their composure in proximity to other dogs.
If on leash and you choose to let your dog greet another dog the leashes MUST remain loose. If you have
tension on the leash you could accidentally create a body language posture your dog did not intend–head up, body tense and leaning forward, etc.–and the other dog could misunderstand. How many times have you seen 2 dogs meet on leash and everything seems ok, then an owner pulls on the leash to leave and the dogs suddenly snark at each other? Count to 2 or 3 and then coax your dog to walk away with you. If you wait too much longer one of the dogs may make the decision, “Yep, I could totally take you”.
DO NOT pull the leash while standing still–same reason as above. Do a little “Ok Skippy let’s go” as you hold the leash low and walk away.
Try to avoid nose-to-nose greetings. Walk your dog in a curve up to the other dog, or walk to the side to form a T, or walk your dog around to let their rear get to the other dog’s head (also known as ‘free butt sniff’).
If there is anything about the other dog’s body language that just doesn’t feel right avoid the greeting altogether. It’s better to have your dog skip the greeting than have a negative experience.