Officially, NM is the abbreviation for nautical miles when used in the context of aircraft operations. It’s not just a good idea, it’s the Law. Specifically, 14 CFR Part 1.2 of the United States Code of Federal Regulations.
Contextually no one is using nanometers in aviation nav applications. Many aviation systems are case insensitive or all caps only so capitalisation is rarely an important distinction.
https://www.ecfr.gov/current/title-14/chapter-I/subchapter-A...