Main Point: The private message box of another user is not your destination to break every rule in existence.
In the past, private messaging has been exempt from all rules - you could flame another user, spam them (Or DoS them), and pretty much anything else. This is not exactly the case anymore.
Private mesaging is meant to be private, as the name states, but do not give a user reason to copy your conversation and give it to a moderator. You can be punished for past conversations via pm if the person reporting you supplies chat evidence.
Private messaging should be your way to get ahold of people you like to talk to, not a way for you to take the player aside and flame them. If you have a problem with another player, this is a good place to go to try and resolve it, not continue it.
Remember that it's better to solve a problem than keep it going. If a moderator has to get involved (Via a player giving them chat evidence of a past pm) it will likely result in a chat kick/wiki ban, so do not let it get to that point.