Never ok. Here's my biggest reference for anti-punishment, anti-smacking:
Punitive Damages
I also DO care if other parents smack. Those are the kids who tend to grow up feeling entitled to disempower and manipulate people around them because of how they were taught to 'negotiate'. We've all worked with people like that - maybe not all of us, but I know for a fact that a great many of us here on BB have, from what I've read over the years of work and people experiences, let alone how our kids come home from childcare and school! I care, because it affects the world I live in and the world my kids will live in with their smacked peers. I care about other kids' welfare.