I think it depends on the circumstance.
I do stay on top of my children's behavior. It may be normal for a 5yo boy to call a girl "stinky" and exclude her from play, but when I heard about my son doing it to his sister, I got right in there and let him know that it was unacceptable. She's his sister, and she'll be his sister for life. I don't care if his friends don't like her, his loyalty should be to her FIRST.
Leaving it up to my daughter wasn't an option, as she didn't care enough to pursue the issue. She would just go and reads a book and ignore everyone. But just because she was willing to let him exclude her, didn't make it right.
I figure it's my job to raise my kids to be kind, considerate and polite to the people around them. And I do keep an eye on their interactions with their siblings and friends. If I think they're behaving badly, I call them on it.
When my son and his sister were spending too much time one summer fighting with each other, I assigned them the dishes on a permanent basis. I told them, "You're going to do everything together until you learn how to get along with each other." It was amazing how fast they pulled together against their common enemy - me!
However, when my children have a problem with another child's behavior, then I do try to help them resolve it themselves - or at least have them participate. When my daughter was 5, a neighbour's 7yo boy pulled down her pants. She came in and told me, so I told her to march right over to his house and tell his mother what he'd done. She did, and came back bouncing with glee. Apparently his mother had been outside talking to his grandmother when my daughter walked up and said - very loudly - "So-and-so pulled down my pants!"

They said, "WHAT??" very loudly, grabbed him and pulled him inside. My daughter felt vindicated and that boy never tried anything with her again.
Two years ago in Grade 6, my daughter was being bullied. It was constant harassment - pushing, pinching, tripping, sneers when she walked by, etc... and it went on for months. She tried to stop it herself without success. We worked with her on being more assertive - for example, saying loudly, "HEY! DON'T TOUCH ME!" whenever he got too close - and we also sent a very strongly worded letter to the teacher and principal making it clear that we would be following up with legal action if this didn't stop. It stopped.
Now that same boy is mooning after her and wants to take her to the grade 8 dance. She's told him to get lost.
So I think there's got to be a balance. I definitely want to be the little voice in my children's heads that says, "Stop, wait, my mom wouldn't like this." But at the same time, I also want to give them the tools they need to be strong, independent adults, who can take care of themselves.
So far, it seems to be working out. At 12 and 14, my children have travelled to foreign countries with family and friends, gotten lost and didn't panic, know how to use cutlery in a nice restaurant, can take the city bus or subway anywhere they want to go, know how to order food and pay for it, and can manage their own bank accounts. I think they'll be fine, even if I sometimes parent a bit intensely.