Like someone else said near the top of the thread...you should have your rings gap set for N2O. Nitrous motors need more ring gap than NA engines do. Other needs: strong spark, plenty of fuel volume at the correct pressure.
The which is safer, rich or lean thing
I say make sure its tuned properly and you wont have to find out. In my experience, I always set mine just a little on the rich side for safety, this is why. N2O is an oxidizer, it is happy if you feed it enough, but if you dont it will keep eating whatever it can to stay happy
like your spark plugs if you are lucky, or your pretty aluminum pistons if you are un-lucky. Keep a close eye on your spark plugs (watching for signs of detonation) and you should be fine.
When folks refer to rich being a problem with nitrous, they usually are referring to overly rich or excessively rich conditions (which is also a recipe for disaster with N2O)...like having fuel ignite where it is not suppose to (bye bye ring lands :-D).
Be safe, and have fun....and definitely get the car running right before spraying it.