This is a question I've only recently had to ask, since I've started a 5 man Priest team on a PvP server just for kicks.

The approach I'm taking is to pretty much leave people alone unless they start something, or if they're obviously ganking lowbies, and to use a KoS mod to track those who no longer deserve the benefit of my doubt!

It's early days, but it seems to be working quite well so far, and since I've always had this damn conscience thing about killing people in one-sided fights, it helps me sleep at night (figuratively speaking ).