Brian, I think Steve may have misunderstood a bit; your stair has a nose, but it's also short of the walls. Steve is correct with wrapping the quarter round.
Continue your quarter round down the face of the nose, and down the riser face below it. I know this starts getting into some pretty small pieces, so one suggestion would be to terminate the quarter round at the bottom of each nose, then begin again behind it with the face of the riser. The thickness of the quarter round on the riser should make the gap at the wall under the nose pretty small and unnoticeable. If it's not very visible that last bit of gap can be covered by using a little woodtone caulking to cover it up.
If it looks like the gap will be more visible, then I'd go ahead and wrap under the nose, too; just use caution cutting those small pieces.