The sod comes with a layer of soil on the bottom. There are two situations when added soil will be beneficial.
1. You have areas that are low or high that need to be built up. Usually, it is building up around a newly installed sidewalk so you do not have a lip between the sidewalk and the grass.
2. You have very sandy soil that needs help retaining water.
Most soil in the area is either a sandy loam or just a loam. Those soils do just fine with sod without additional topsoil. Added topsoil does not hurt your sod, and it can do nothing but benefit it, but it is not necessary in most situations.