From what I know about the subject, I would probably agree that the first bomb was not a mistake.  If the bomb had not been dropped, and the US had to invade mainland Japan, there would have been many more deaths than those lost by the bomb.  Also, the development of the bomb was just a matter of time, and if it had not been dropped on Hiroshima, it would have been dropped somewhere else.  Who knows where that might have been.