It's funny because you even see it with very small kittens...my kittens are now 4 weeks old and they are all play fighting with each other and their mom, Zuri...it is hilarious to watch at this stage because they are so little...and they do learn to back off when bitten but then run right back in and start all over again...so as others have said...unless it is so rough that one cat is terrified of the other or drawing blood, I would let them work it out.