Most bombs will airburst and are highly efficient. Airburst typically will have minimal fallout. Im not even sure fallout will be the main issue in a modern nuclear war. Obviously the places directly hit will be irradiated but there may not be much affect on everything else. The first bombs they dropped on japan were pretty shitty and only a fraction of the uranium actually detonated. Modern bombs are likely to detonate 99% of the fissile material.