Whether you have to get it smogged or not, you should figure out what's causing that high NOx reading. As people have pointed out, that's a sign of high combustion temps - which could mean a lean condition on all cylinders, or maybe just one or two. If you have a lean condition, you could burn a piston eventually, so it wouldn't hurt to do some troubleshooting.
Also, some of those additives out there will increase NOx levels - so make sure you're using high quality gas with no crap in the tank. By the way, I always wondered why Shell was advertising that "Nitrogen Enriched" gas as a big benefit. While nitrogen (ammonia compounds) can be great cleaning agents, I'm not sold on using them in an internal combustion engine that's supposed to minimize oxides of nitrogen production. There's enough nitrogen in the air to cause problems, why add more?