For me, the risk of having a poorly socialised dog (with people and other dogs) outweighed the risk of parvo. It's all about the precautions you take.
For us to have waited until she was 18 weeks (she had her first needle at just shy of 10.5 weeks and her 2nd at 16.5 weeks) that would have been really really bad for her.
To give you an idea of her size, she was 18.38kgs at her 16.5 week weigh in at the vet when she got her booster. Her adult estimated weight is in the vicinity of 55+kgs so a poorly socialised dog is not something I wanted to risk. At all. She is 4.5 months old and 25kgs (all legs, slim and tall) she meets small dogs with a bowed head and waggy tail and small dogs and cats she lays down in front of. I am glad I didn't wait.
It's all about making educated decisions for your dog. I think the risk of you bringing parvo in to your house on your shoes is greater than your puppy contracting parvo walking on a lead in the backyard while toilet training.
Asha - 22/05/15 Wolfhound cross a Dogue De Bordeaux X American Bulldog.