Jobst Brandt addresses the topic succinctly:
It has often been suggested that looser spoking will improve the cushioning of a wheel, for instance one used on rough roads. Because the elasticity of spokes arises from the material properties of steel and is not affected by more or less tension, no change in ride quality can be achieved by loose spoking. Spoking with less than optimal tension only forfeits strength and durability. (Brandt 71)
In my opinion spoke tension should be as high as possible, which is usually determined by rim strength. A wheel built with high tension where the spokes are in balance and stress relieved is the strongest and most durable wheel. Instead of building to lower tension, build to ideal tension with lighter spokes. Lighter spokes have greater elasticity, which absorbs road chatter.