I let mine go relatively crazy and just keep an eye on things to keep it from escalating. They're allowed to knock the crap out of each other as long as it's clear they're both willing participants and having fun. But I will step in and enforce a time out if someone needs it and the other is still trying to be a lunatic.
There are also certain 'moves' I don't allow. No biting each other's legs - like herding - when one is trying to walk away, no biting down and hanging off each other's skin or tugging on it, no full on mounting each other (a paw over the back is fine, but absolutely under no circumstances do I allow them to mount each other), no pinning each other to the ground (they can roll around but the second one of them tries to pin the other, they're called off)...basically I stop any sort of rough play that has real potential to start a fight either by trying to dominate one another, or potentially really hurt one another causing someone to lose their cool.
I also watch general body language closely to monitor how they're both feeling about the play so I can put a full stop to it if I need to. And I stay within 5-10' of them while they're wrestling really hard (mind your knees and shins! LOL) so I can enforce things like not pinning each other and no hanging from each other's skin. Plus, if I were to miss something and need to pull them off each other, I want to be close, not 100' away. Damage can be done in the blink of an eye.
I tend to be vigilant and cautious with how rough I let my girls play because they've had a couple arguements and it's not fun when they cross that point from really rough play into throwing down the gauntlet with each other.
It was fine when Fiona was younger but as she's matured both her response to rough play and her style of rough play evolved. She started pushing a bit with her actions and responding differently to other dogs. It seemed like a subtle shift in her behavior but even subtle changes can have a big effect between dogs. So I created firm guidelines and rules for their play for everyone's safety. They're both respectful of the rules and it helps keep everyone on friendly, happy, playful terms rather than allowing them to tempt a fight.