I only single-box on a PVP server (Human Warlock), but, I generally:

0. Always attack people on my KoS list if it's prudent to do so. If they are guard-humping or if I'm grossly outnumbered then sometimes I'll hang out nearby and wait for them to move. I do occasionally fearbomb raids outside SSC and things like that, though. Managed to drown someone who was AFK before that way.

1. Don't attack if either of us are just traveling through. I won't generally dismount to gank someone grinding something I've got no interest in, nor will I jump on people who ride past me. If I've set up camp somewhere and they stop long enough for me to finish whatever pull I'm in, or start targeting me, then it's on like Donkey Kong.

2. Attack anyone competing for things I want. If I think there's a good chance of me losing the fight, and whatever I'm after isn't really competition limited I may move on and get what I need, and *then* come back and attack them. But if it looks like they are going to initiate combat then I'll jump on them first anyhow. Initiative > any other factor in a lot of cases. Especially when you can fear someone into some helpful mobs.

3. As a general rule I don't attack greys unless otherwise provoked. I have even on occasion "helped" them by putting seed of corruption on their mobs!