The most important thing
Socializing. Yes, I do believe that it is the most important thing for a puppy. When, where and how?
Socializing - social;
Early neurological stimulation seems to be important to many breeders; while others don't adhere to the strict process. In my opinion, the steps can be helpful if it is the only way to get the puppies handled at a young age. What I believe to be far more important is handling and just general life stuff introduced at a young age. I like to look at wolves in the wild for much of where I form my opinions. Wolf pups do not receive a set of formal stimulation steps when they are very young. What they deal with is life in it's many forms.
The rule of 7s - great socializing goal. (I love this)
Handling puppies at a young age and introducing them to many different things is so very important. When Elsa arrived, she very literally came out of her crate and announced her arrival! "I'm here, let's go." She had been exploring in different environments, playing with adult dogs and been handled regularly. She was sure footed and extremely advanced as far as physical capabilities. Part of that of course had to do with her genetic makeup but a great deal had to do with what her breeder offered the litter.
I have met litters who live their life in a whelping box. Their first experience to the great big world out there is 8 weeks of age. These puppies are at a disadvantage; so many weeks of missed opportunities have passed by and are forever lost.
Socializing is a balancing act. If a little is good, a lot is not better. A little socializing each day goes a long way. Puppies need to feel secure and if they are socialized too much, it can backfire. But the lack of socializing can be detrimental to a dog's entire life. Sure they can get socializing in once they join their forever family but there will always be those missed weeks where they sat in a box un-stimulated.
Puppies who are placed in a kennel type situation and not given the chance to experience home life, people and dogs are at a disadvantage. When choosing a breeder and/or puppy you should ask or see where and what the puppies have spent their weeks doing.
It makes absolutely no difference what breed or mix puppies are; they all require the same life stimulation. As the puppies grow, so should their socialization. New people of all sizes, different dogs, different surfaces etc etc. A puppy only lives with a breeder for 8-12 weeks typically; much can be done in those few weeks to help get those puppies on the right track and ahead of the game for the life ahead of them.