Socializing a puppy is same as socializing with human! The more you are together, the more progress you make. Introduce your puppy to everything - people, places and other animals. This is the key to social development. Otherwise, it can be stagnant and even regress. The idea of socializing is a confident dog that isn't too shy or too aggressive.
Like with anything in life, safety comes first. If your puppy needs that last vaccine and your Vet says not to take your dog out until then, your Vet is correct!
But that doesn't mean you should leave your puppy at home.
Common sense is the key here. Plan safe outings. Take a puppy class: Good trainers know the risks and work to minimize them by keeping the training area sanitized.
Teach a Life-Saving Skill Set
Why take any chances at all? An unsocialized dog — whether fearful or aggressive — is at a higher risk of ending up in a shelter with little chance at being adopted again. Some experts argue that, in the long run, behavior problems kill more dogs than parvovirus does. Perhaps that puts the importance of proper and safe socialization into perspective.
Limit the Paw-Holding
Unlike wolves or coyotes, dogs are genetically predisposed to become part of human society, but it's not always easy. So socialize, and remember that the world is full of scary things, especially to a little puppy. At times, even the boldest of them may become paralyzed with uncertainty, especially when faced with something they have never seen before.
Your response to this fear is very important. Don't soothe your pup. Petting him and saying, "It's OK, baby" (or something similar) gives your puppy the idea that being scared is OK and that you're rewarding him for the behavior. Instead, be matter-of-fact and encouraging. Let him work it out, and when he takes that step forward, praise him for his courage.