I always stop when I'm in the car. On the bike, though, I won't stop if I'm sure the coast is clear. If a big van is parked too close to the intersection and I can't see around it, I'll slow to a crawl and make my mind up. If someone is coming, obviously, I won't "steal" their right of way, more out of politeness than for fear of being hit.

Basically this comes down to bikes being invisible to The Law, and that's a double-edged sword. But when it comes to why it should be ok to run "empty" stop signs on a bike, Newton's laws provide a pretty good answer.