HMMMM, great question...
I think there needs to be a balance.
When I was growing up, in a middle-class, dead end street in SoCA, if there was a disagreement between kids, it could easily escalate into an actual fist fight. There would be periods of getting along, with the occasional 'now were enemies' for awhile. And our parents were not involved with these changes.
I don't think a complete hands off approach to letting kids sort out their differences is going to teach kids how to resolve conflict when the inevitable conflict arises. We were certainly not taught any of these life skills, in my generation, by parents, how to respect each others differences and decisions.
I have invested a lot of time with my own children...teaching them how to compromise, how to negotiate with one another, how to politely and respectfully play together and with others...basically they are well-mannered kids.
I personally don't tolerate spoiled, whiny, selfish, cry baby kids very well, even as an adult.
There is a difference between having a bad day and a friend takes his frustration out on a friend versus the same kid having a continual behavior problem that negatively effects the friendship.
In the final analysis kids will learn more bad from a bad kid than they will good from a good friend.
I am truly not a helicopter mom....ask any mom who knows me...I'm probably a little too cavalier about my kids independence....but when I see behavior problems emerging, like lying, cheating, stealing, hitting, uncontrolled anger, I most certainly will talk to my own kids about the long term viability of that friendship. Meaning it's time to search elsewhere.