In a chemical attack, the contaminants are typically distributed in an aerosol that is heavier than air near the release of the chemical. As such, it will settle to the ground.
However, as the vapor plume is carried down wind, the concern about settling in low-lying areas is reduced. With this in mind, the best room for sheltering-in-place is the room that is the most convenient for your family to quickly get to and seal that is large enough to provide appropriate air for several hours.
With regards to a radiological or nuclear attack, the more shielding, distance and time you can take advantage of after such an attack, the less exposure you will face. The additional distance from the blast of a nuclear weapon provided by being below ground also offers an increased level of protection.