Views on when to/not to allow a dog to be social

LOL, I have Collies the Will Rogers of the dog world. To them strangers are friends they haven't met yet. My Collies get a lot of interaction in our travels. By the time Sawyer was 2.5 years he had been to 29 states. Very seldom do I travel without them. Sasha is a little more skittish and reserved. She waits for Sawyer to make first contact before she joins in. That said I'm kind of selective on what dogs I let them interact with.