I have a rescue dog that is not properly socialised with other dogs so I really think it's important to let the puppy meet other dogs of different types. My dog accepts certain kinds of dogs, those that look "normal", but not dogs that are different in any way (for example really big dogs, dogs with a lot of fur or dogs with very flat noses). If the puppy learns from a young age that dogs comes in all shapes and sizes it is more likely to be friendly with other dogs.

With previous dogs I have socialised them just by bringing them with me almost everywhere: to visit friends, to the dog training club, to town, to the country, on the bus, on the train etc.