The location you choose for your bomb shelter should be one which gives you the greatest protection possible. Just placing an underground bomb shelter in your back yard is not enough. You need to find the most optimal location. In doing so, you need to consider the terrain, water levels, and distance from other structures. This includes location of sewer and electrical lines.
Essentially, you want to have as much mass between your bomb shelter and the attack. After identifying potential nuclear bomb targets and where a likely blast wind will come from, determine a bomb shelter position that will serve the greatest protection. The lower edge of a hill, for example would be an excellent place for your bomb shelter if a prospected blast happens in the distance on the other side of the hill. The idea is use common sense in determining where an attack might occur. Then, build your bomb shelter in the best possible location.
In considering the fallout aspect, just know that its going to fall everywhere; no hill or mountain would reduce the amount of fallout since it falls straight to the ground like snow. So, you just need to make sure you put your underground bomb shelter deep enough – 36 inches or more below the surface.