I stop and look both ways. At least as much as I can without clipping out.

It's the law. It's a matter of respect for other people using the same road.

But most importantly, long experience has demonstrated that I am anything but infallible. Because of that, it's foolish for me to roll through stop signs counting on myself to make no errors in determining whether it is safe to do so. If I stop at the stop sign before proceeding, even if I were to miss the vehicle coming the other way, they would likely see me and stop. If I roll right through ... well ... I'm more likely to get creamed.

I've noticed that a lot of such accidents are really the product of multiple failures. One person rolls through, thinking it's safe. And so does the other, either distracted by a kid, a cellphone, a babe in a bikini, or what not. Either failure by itself is not the cause of the accident ... but combined together ... pow.

I dunno. Frankly, I ride too many miles not to want to minimize my risk exposure every time I ride.

The only exception to the rule is if I am in a good sized paceline. In those situations and depending on who you're with, stopping itself is risky. And that's one of the reasons I'm not enamored with pacelines.

Edit to add: I'm amazed at the number of "I stop" responses here. Frankly, 95% of the cyclists I see appear to ignore stop signs altogether or give them a casual glance.