I also play it by ear, but my rules are much more lax and more like suggestions:

1. No killing during an escort quest.

2. No camping unless I've been camped (and even then it's short term at best).

3. If it's with someone in "my area" but I don't want to really start a fight, give them a "warning shot" (something that'll scare them, show them I mean business, but by infrequently attacking in an obvious manner, lets them know that they need to move on).

However, this is also based on server health. The alliance on my server were dying at Cata's launch, so my guild discouraged world pvp just to give the chance for the alliance to thrive a bit. If they started a fight, we'd bring enough to fight back, not massacre them (most of the time). However, when they started to pick up but, overall, not pvp, we came down on them hard, including lowbies so they would bring their main/friends/guildies. The overall goal is to keep world pvp alive on the server, rather than get my jollies (unless it's chain deathgrips across zones- that's just fun ^_^ )