Someone suggested a while back, on one of the Internet threads I follow, that the "nervous" shoulder check might actually keep cars a little further from you as they pass. I noticed that, because I'd started to get the same impression myself. In fact, I'd started doing it deliberately as a means of making cars give me a little more space.
The comment came up in a conversation about eye contact, which I think is important. The more you make eye contact with drivers, the more they see you as a person rather than an object: that's a no-brainer. Better yet, if you make eye contact and communicate - a wave of the hand or a hand signal or a nod saying, "no, you go ahead," or "I'm turning that way" you're more likely to have a, well, human interaction with the driver.
(Even when it's not a particularly friendly one: yesterday, as I was trying to navigate a construction zone and the lane was narrow, a couple sped past me about six inches from my leg. I shouted in fear - then noticed that their window was open and I could assume they'd heard me (for once.) I also had the chance to catch up to them (it was a construction zone, and passing me had gained them exactly zero ground) to shout "That was WAY too close!" through the open window. My eyes met the driver's. He looked... startled? Lost for words? Maybe angry, maybe defensive, I don't know. He was leaning toward me, like he wanted to see me too - I got the feeling he didn't quite know how to react. Good, I thought. With luck he'll think about the next cyclist he passes.)
Anyway, I had found, just before reading someone else's observation of the same thing, that when I was coming toward an intersection, swinging out to avoid a pothole, or even just moving fast, down a hill or something, and I looked back over my shoulder a little more frequently, it seemed like cars gave me more space.
It's totally unscientific, I know. But it does make some kind of sense. The drivers see me turn to look. The change in state - from "faceless cyclist" to "person on bike" - is subtle but I think it's important. They see the side of my face, not the back of my helmet. They see that I'm aware of them, in the way that I assume a "faceless cyclist" appears not to be. There's a subtle interaction. They think, subconsciously maybe, about the relative positions of their car and my bicycle. And they probably interpret my shoulder check as a signal that they've made me nervous, which puts them, momentarily, subtly, in my shoes. And they back off a bit.
At least, that's how it seems to me.