depends who's fighting. The Tokyo region has something like 35 million people so that would be a popular target, so I'd give it a miss. Hitting London (about 14 million) would wipe out a lot of bankers and everyone would have to revert to barter. LA has about 13 million and is home to the giant Hollywood propaganda machine, so it wouldn't be a bad idea to nuke them. Avoid conurbations, therefore.
People at the end of The Chrysalids headed for Sealand, which agrees with belladonner's suggestion. Otherwise further out in mid-Pacific - Tahiti, Fiji or Hawaii perhaps. You might get tired of drinking mai tais, though.