To intervene or not to intervene

Yes, I know, touchy subject...but what parenting topic isn't?

Playgrounds seem to be the testing ground where coping skills are developed. Kids interact in free play, utilize their imagination and creative thinking skills....and deal with differences in that direct, often brutally honest way.
We parents who watch our kids at play often feel that slight edge, like "Will my child shove another kid out of the way in their determination to be first down the slide?" "How should I best handle it?" "What if my kid is the one being bullied?"

I for one am NOT comfortable stepping in to police another child's behavior and often simply remove or redirect Henry from a potentially confrontational situation, saying something indirectly like, "We all need to play nicely" unless I see actual aggression or physical danger and then the situation definitely calls for an adult-intervention.
 And then I see those who have no problem whatsoever correcting another child's behavior, whatever it is, regardless of the parent's presence. Seriously? Give me a chance to respond before you jump in and give MY child a lesson in manners.

So when Henry was recently playing with some kids a few years older than him, I was the only adult nearby, off to the side, watching to make sure he didn't wander away or decide that it would be AWESOME to dive from the tallest surface. They were all gathered around and I heard Henry getting all excited and start his familiar 3 yr-old stuttering, "Um, um um..." Then I heard one kid laugh and repeat Henry's "Ums."

My Mama-hackles rose. Who the heck is this kid and how dare he make fun of my Henry?! I started towards the kids when I saw Henry laugh delightedly and continue playing, absolutely unfazed. OK, he's not hurt or upset and he's never been shy about coming to get one of us if someone's pushed him or said something mean.
Correcting that kid would draw attention to the fact that he mocked my son, even if Henry didn't aknowledge it. This kid's parent is NOWHERE around. So I decide to stand closer and pointedly watch them to determine if I do in fact need to intervene.
Nothing more is said and Henry's being included in the group play.

I decided my closer proximity was sufficient and I didn't say anything in this case. I'm sure one of you will think I should have. Maybe you're right. But I also realize that if I intervene in every single incident, no matter how minor, how will my child learn to deal with others when he's apart from me? I can't be there every second to protect my son from every single situation, no matter how small (and no matter how much I want to). If he had acted hurt or upset, there would have been no question that I step in- he is only 3. But at some point too, he has to figure things out for himself.
Where is that line, anyway??!! Where is that elusive spot between "Helicopter Parent" and "Man Up"?