It depend a lot of the situation in your country for these diseases, here I simply have no courage to let my dogs touch outside my garden or meet different non-well-known dogs untill they complet
4 months, having the basic vacination all done and already working, with perfect imunization.
I keep socializing my dogs at house, with different sounds, exposing them in different situations, with a small trip by car little by little I made them know the crazy side of the city in a safe way.
Also I use the house of my friends and parents for make the pup know a different place, with different smell and also with different dogs.
Anyway my dogs are separated in two "packs", so the young live with one but often I make the little one meet the other "pack", for the pup its already a different dog.
As I live almost in the second centrum of my city, its common they listen hard sounds suddenly, like shots or explosions, when a fotball team wins its sounds like if the world would fall or as if we're in the middle of a war.
I can say that the real socialization here starts after 4 months, that when the pup can walk through the city, meet a good size pack of different dogs in park, play in avery different place and you dont need to worrie about distemper of parvo.
But they've learned everything previously already before, at home, even if in one small exemple, they already knows how to react and how keep going without problems, so the socialization can be done perfectly after the 4 months, as the basic has been already done at home.
When you have a dog (or a pack) with excelent behaviour and very well socializated, you can also use his help to teach the pup, it can turn your live very easy as sometimes you only will use the pups ability of learn observing in your favor.