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

For us it's more about the handler than the dog. People that use the extended length leashes and let their dogs roam at the end are avoided by us. My dog seems to see these as a loose dog and goes on alert. If a dog is properly leashed and controlled by their owner we have no problem letting them socialize. They can run free(in a confined area) after they have met and show they can get along.