I started taking Polly everywhere with me as soon as I brought her home (7.5 weeks). With her being so naturally timid, I knew the risk of having a nervous/reactive dog was higher then our risk for disease.
So, she went to carmax to help us pick our our van, to the feed store, to Petsmart (in arms until 10 weeks when she started puppy classes), to the high school where my husband works, to as many friend's houses that we could visit, etc. She was in the car with me every single time I went anywhere and if she wasn't allowed in, she waited in the car (she was an early-spring pup). We started at the dog park at 4 months after her final shots. We started with occasionally doggie daycare at 7 months but I wish we'd started earlier. She was nervous at first.