Plexus - don't
Plexus builds up a waxy coat on surfaces it hits. This stuff is amazing for plastic convertible windows, paintball masks, and any other exterior plastic surface. But since your nav screen doesn't see the outside, I'd stick to a lens cloth. The best will come from an optician (eyeglass shop) and cost $5 to $10. Ask for a reflection-free lens cloth. They are washable so you can reuse them forever, and totally lint-free.
if the screen actually gets dirty, I agree with the recommendation of screen-wipes. CompUSA (bleh!) carries them cheap.