I pass on the left when it's possible and not dumb. Cars pass us on the left, and for the most part we're always ready to be passed on that side. So it makes more sense.
I was riding in Queen Anne when this hipster came into my lane in front of me from a side street. He was going a lot more slowly than I was, and he was weaving back and forth on the left side of the lane, like he might have just finished some PBR. So I passed him on the right, but gave him as much space as I could have, putting myself in the door zone. I heard an explosion of swearing behind me, with "don't pass me on the ****ing right" somewhere in the outburst.