I would sit on the bench and just supervise, but my very cautious and shy daughter won't play if I do that. She likes us right there. She is 5, and I don't have any worries about physical stuff. Like others said, when she was younger I was close due to behavior of other kids...I don't why, seems like lots of negative behaviors happen at the park. But, since she was very little, she is more comfortable around other people if we are right there.
And, I have felt like the more "let them run free" parents have judged me a few times thinking I am "hovering" because she is older and I am still standing among the play things and not on a bench, but, like I said, that is the only way she will go and play. If I insisted on sitting on the bench, she would sit on the bench with me and watch.
So I guess there is not definitive answer, the personality of your kid has a lot to do with it, in my opinion.