I play on an RP/PVP server, I re-rolled here as soon as this type of server opened (at first, RP was only PVE).

I don't have rules, I go just by the feeling.

Usually, I don't attack first, which used to be a big issue when playing one under geared character (casual player). It is of course less of a problem now. When attacked, I kill. If I get killed, I come back for revenge. I can decide to attack a player because he has a name not in line with the RP chart ("Psychokilla", "Röxxxör", etc ...). I systematically kill every Undead rogue I can see. I also use a KOS addon and systematically kill whoever appears on that (can have been added by alts or guildies). I sometimes help the Horde, it they are Tauren (I like Tauren), or a healing class soloing in a hard spot.

When playing my Druid + 4 x Hunter team, helping can be tough because of the sometimes erratic behaviour of pets due to targeting lag. I learned to not click on a friendly Horde character but just mouse-over him so that 2 out of 4 pets won't assault him when I engage my next mob. It happened quite a few times though. Same thing whit /autoshot, it can be tricky.