I think this depends case-by-case. Ziggy has absolutely no herding instinct, and I've heard of other Corgis that don't as well.
While I'm not saying that you shouldn't be prepared in any case and know how to dismantle a dog fight before owning a Pittie, I don't think that JUST because your dog is a Pittie that they can't be loose at a dog park. I've had a Pittie, and he was so incredibly gentle, even when being terrorized by other dogs or by kids. He wouldn't have harmed a living soul, no matter what/who/where/when for whatever reason.