i just know from personal experience that (inb4 anecdote police) that dogs get defensive and can act aggressive towards other dogs while on a leash but are much more playful while off the leash around the same group of dogs. But I agree if you're nervous how your dog would act it's best to not bring him off leash and imo bring him at all. That's good you watch his tail it's the easiest indicator to him being playful vs aggressive I just wish all other dog owners knew to look for that.