I personally am not a fan, however, if you want to do it safely here are a few points.
1. Knife play is a form of edge play, so it is inherently dangerous and should therefor be treated with the proper respect. Do not just pull a person off the street and ask them to cut you. Knife play, and all edge play, should be done with someone you either a)know very well and therefor trust implicetly to take care of you before, during, and after, and someone who knows what to do should something accidentally go wrong, OR b) someone who you have seen to have the necessary skill set and that you can therefor trust to know what they are doing.
2. Make sure to not cut too deeply. The least pressure in the wrong place could be fatal, so when starting out make sure to use smaller blades that can be more easily controlled.
3. Make sure you are in a secure place where no is going to walk in unexpectedly. If the person with the knife gets startle at the wrong moment, it could be very dangerous.
Be safe, and smart about it. If you have open communication with your partner, and go over it first, at least in the beginning, you'll both be safer.