My kittens, Penny and Duffy, are play fighting all the time. I only jump in when it's obvious that the fur might start flying. They can get pretty roughty, but so far haven't hurt each other. They jump at each other, chase each other up and down the stairs, lay in wait, wrestle, and roll around. They don't appear to be using their claws on each other.
I watch Penny more closely, because I've noticed that if Duffy does something she doesn't like, or if he gets too aggressive with her, she'll bite his ear when he isn't paying attention.