When DD plays at others' houses, she follows their house rules. Same goes here. We are very lucky that her two best friends have parents that have similar rules as we do. There have been no issues. The kids get along great and can resolve their issues on their own. I step in occasionally to remind DD about her tone of voice (she can be incredibly bossy at times so I pull her aside and remind her to be polite), but it is only after I can't stand listening to it any longer. I used to have to do it a lot when she was younger. I guess this is as 'helicoptery' as I get!
She is well versed in what we consider safe behaviour and has brought herself home on a couple of occasions because kids were not behaving or she felt uncomfortable.
If she was going to have a playdate with a new friend, I would have to meet the parents first and at least see their house before I would be comfortable. After that, I let her go.