A mechanic at a nearby shop builds wheels with high or low tension.  He acknowledged high tension is the conventional approach but thinks low tension offers superior comfort and is suitable for most applications.  I can’t agree.  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.  Such a wheel is not less comfortable.

Ride quality is not a concern to be dismissed but I think cushioning is better achieved with tire choice and tire pressure.  First look for a quality tire with a supple casing.  If more comfort is needed, consider a larger volume tire that can be run with less pressure.  Mountain bikers know tire pressure is as important as suspension when it comes to ride quality.  Indeed mountain bikers have embraced tubeless wheels, which allow very low pressure.  (Tubeless technology has made its way to the road scene recently though product selection is a little weak at present.)