Lash will effect the opening timing and closing timing of the valve. More lash equals shorter time valve is open, less lash, valve is open longer.
A valve itself can cause pinging, ONLY if it has a carbon build up enough to glow just as Jerry explained, usually around the edges. I could see how to much lash could possibly cause some pinging if the valve doesn't open enough to fully fill the cylinder, thus causing a lean condition, but, it would have to be pretty far off I suspect.
