Most high-end deck projects use closed risers to finish the stairs.  Open risers appear cheap and unfinished.  They are also often in violation of IRC code, which maintains that they meet the same requirements as guardrail infill.  Stair risers may not have openings present that would allow a 4” diameter sphere to pass through.  Most risers are made from deck boards or fascia.  Make sure the material is heavy duty enough to resist feet kicking the risers.  If you are using 1x material you may want to install blocking behind the riser board for added strength.  Low voltage lights can be installed into the riser boards in order to meet the IRC code for stair illumination.  Most stair assemblies use a nosed tread that extends beyond the riser board to protect it from being kicked loose from above.