Each puppy is different, some puppies are ready to roll right off the bat while others need a bit to acclimitize. It really does come down to genetics as Lies has already mentioned.
Personally I take the pup out everywhere, getting them used to car rides, meeting dogs that I know are safe, and just learning what life is. If the pup is hesitant then we slow down, if they're charging forward and ready to rock then we continue on.
My only worry is diseases so anywhere where dogs frequent I carry the pup so they can see/smell/hear everything but from a safer distance. Once the shots are completed I take away all restrictions and just watch the pup and step in if needed if they're uncomfortable with something.
Jasmine - Female Miniature Poodle - born Aug 15, 2010
Loker Delgado Von Stalworth - Male GSD - born Jan 26, 2012
Koda & Zazu - 5 year old male cats
Alex - Male Cocker Spaniel (rescue) - RIP Cuddlebug 2007-2010