So, if I understood your answer correctly, you mean that one should not OOC-join a fight just to stop a griefer (unless its truly blatantly obvious that it is indeed griefing).
Let me be more clear. If anyone ever attacks you, or attacks your friend, or anything that gives you an IC reason to defend yourself or others, then you can always intervene if it's IC to do.
If it's someone you suspect of being a griefer, do not intervene for OOC reasons unless you're -sure- they're griefing. For example, you see Johnny Halforc pummeling Joey Halfling, and you don't know either of them/your character doesn't have any reason to get involved. Running in to smoke Johnny to OOCly stop him should only be done when you're sure he's griefing, because if he's doing it legitimately and you attack him for OOC reasons then it's you that's causing the issue. Determining whether someone is a griefer is usually pretty easy, if you see a low level PC randomly attacking people in front of NPCs, send a /dm message asking if it's being supervised and if nobody responds, then stopping him should be fine. In cases of griefing, trying to get a DM to come IG via IRC is recommended as well.
Fortunately this issue hardly ever comes up though!
But, in that case, is it still ok to IC-join an ongoing fight in front of NPCs without asking for DM supervision? (since it should be safe to assume that the first attacker has made sure such supervision is already present.)
Again, if you or your IC friends are being attacked you can always defend yourself no matter what. If it's a situation where Player A attacks Player B, then runs off, and you want to chase/stalk them around and gut them, you'll still need to get a DM before attacking in front of NPCs.
For example, you're hanging out inside the Gate, when a PC pops out of invis and starts hacking your pal to bits. You can respond to this and defend him without checking for a DM, since it was the responsibility of the initiator to get a DM. If they pop out, kill your friend, then go invis and you see them inside Town Hall five mins later, you'll need to get a DM if you want to attack them.
The responsibility for getting a DM has always been on the person initiating any combat.
It seems complicated but it's actually very simple.
A: Get a DM before you -initiate- combat.
B: If you're the one on the receiving end of an attack, you can defend yourself fully, and it is not up to you to get a DM before defending yourself.
C: If a PVP combat ends (ie the immediate fighting/chasing stops), make sure you have a DM before starting it up again, whether you were initially the attacker or victim.