If the runner was clearly going to be out and the throw did not bounce, then I usually score it as an error on the IF and do not credit the runner with a stolen base. If the ball and the runner arrive at the same time and the IF drops the ball, I would score it a stolen base because there is no certainty that the ump would have called the runner out. And of course the most obvious are the ones where the ump has already signaled the runner is out but then changes it when he sees that the ball is loose and the infielder dropped it. The same standard applies to catchers who drop throws from the field on plays at the plate and the run scores--the catcher gets the error.