What are the grammatical rules for using "�" or "�" instead of "N" or "n"

There really isn't a law or rule. They need to be treated as two separate letters. They are two separate letters in the alphabet, just as "l" and "ll," and need to be treated as such. It would be like mixing up "pero (but)" and "perro (dog)." There is no rule. That's just the way they are spelled.