I don't "veer" left, I push left a bit, and never really more than to the right tire track. I only do that if it's pretty clear that someone doesn't see me. It's been said here before that some drivers "see" only what's directly in front of their car; if you're a foot to the side of where their car's body will go, they tune it out. Getting into that zone for a second wakes some drivers up, but I never do it unless the car is far enough back that I can push left then come back right again before they reach me. I want to make sure they see me, not try to actually act aggressively towards them.

I think this depends on your riding situation. Almost all of my riding is on rural roads, and most drivers see me several hundred yards before they get to me, and there's rarely oncoming traffic. If someone hasn't started to move over by the time they're 100 yards back or so, they need waking up.

I do this in in-town traffic too but as you say, tolerances are a bit tighter there. If traffic is coming that often, it's usually best to just go ahead and stay as far left as you care to go, for me, right tire track.

Luckily, traffic around here is extremely courteous, in fact they bug me sometimes because they're so courteous; it's like they're treating me like a child. This morning I was at the right edge of a 3-foot-wide clean shoulder outside a wide car lane on a 50 MPH road, and two different cars slowed down and followed behind me at my speed for about 20 seconds until oncoming traffic cleared. I don't know WHAT that was about. They could have passed me in the middle of their lane and left a good 5-6 feet between me and them, and certainly they could have seen that after slowing down to my speed and following for a few seconds. Though I guess it's better than being buzzed, I keep thinking "I don't need 15 feet of clearance, people."

I can only assume that they think that all cyclists are totally unpredictable and might veer in any direction at any time.