Honestly, I think the main focus you should have as far as socialization should be all the persons, places, things that your dog will normally be exposed to. And do your best to keep that dog's confidence high and cocky through all interactions and experiences that will be pretty common in his life.
With Bertie - that meant going for walks in the rain, going outside and sitting under the eaves during thunderstorms, that means swimming in icy water, sitting in the crate at class, sitting in the car while I run in to the gas station or store, that means going to classes or training places (open floor) ever week from the time he comes home onward, going to petstores every week, going to various lakes, hiking through the woods and across fields, learning to visit with elderly and little kids of our neighborhood without jumping, learning to ignore "non-family" dogs unless I allow it and then not doing anything else besides nose touching, going to the vet every week for weigh ins and learning to like going there....
All of the different things out there that are part of these activities - he was exposed to very early. I think he's a pretty confident dog so far.
