There are also very few people I'm biased against in the forums. I also don't tend to recognize people in the game as forum users. I don't recall my character ever being outed in the forums either. So I don't really have an informed answer to this. Some players do change their behaviour based on who they think the player is, so there is a reason for rule 4 to be there.

If you treat people differently based on who is playing the character, then your own RP is going to suck when interacting with the character. That's not a good thing. So if you treat people differently based on what you think of their assumed player, that would explain people's polarized views about the quality of your RP which I've seen in the past. Everyone is better off if you give people a chance and judge the character only based on what you see in the game. If the game experience is positive, the relations with their forum persona might also improve after the characters croak. That said, you are generally better off not knowing who plays who. Although not everyone is affected by the knowledge, your experience is likely to be more immersive if you don't know the player of the other character.