I did not use anything under the riser - maybe one day I'll regret that, but I doubt it. The riser is resting on finished hardwood flooring. In places that framing rests on concrete, I used pressure treated lumber.
The 8" joists in the riser are: cheaper; easier to handle; will never touch the ground, so they won't cause the riser to rock or be uneven; and allow the air cavity of the riser to be a single continuous space, should I decide to try to use it for bass trapping. The outermost two joist bays on each side of my riser were sealed with some OSB and 6mil plastic and filled with sand, because I'm putting subwoofers on them. The middle joist bays were left mostly open, but I did suspend r-13 insulation on hangers in them before I topped them with subfloor.