All out nuclear war in todays world is highly unlikely. The USA knows this, Obama revised the US nuclear strategy, Stating that the USA will not use nuclear weapons on any non-nuclear capable state that complies with the Nuclear non-proliferation treaty even if they attack the US with biological or chemical weapons. Although this excludes Iran and North Korea.
http://www.nytimes.com/2010/04/06/world/06arms.html?_r=1
Rogue States and terrorist are far more a threat nowadays, where preemptively striking them wont make much sense.
Hiroshima and Nagasaki I dont really see as a mistake. If they didnt drop the bombs, how would you explain all the loss of Allied lives, if we had this bomb that we chose not to use, that could of ended the war, but instead we decided to invade Japan.
OT: A preemptive stike depends on a lot of things, you have nothing to gain by nuking a city preemptively. You would most likely preemptively strike another nations nuclear sites that are hard to kill, like missile silo,s but even bunker busters could do the job. I find it hard to see a scenario where you would absolutely need nukes as a preemptive strike.
So i cant really rate in this poll, just thought id give my opinion.