which fuel should I use reg vs premium ?

Did you check the other possible causes for too much NOx? Recent tune up? Oxygen sensors replaced recently? My wife used 89 in her 1990 Camry with no problems, too high of octane will lead to carbon build-up and is basically just wasting your money. Suggest putting some throttle body cleaner or fuel system cleaner into the system, or octane booster, and running the engine hard before taking it to the emissions station next time, that should help lower NOx.

According to my 91 Camry shop manual, a faulty EGR valve will cause high NOx. You could buy new one from Pep Boys for $several hundred, or you could get one at the local salvage yard for $10! That's what I did. My Camry has 194k miles and I got the EGR valve off a 90 Camry with 90k. I cleaned it with carb cleaner and it was good to go.