Word's meanings change over time. Jap is still offensive, so don't use it. Just think what are some of your cultural/ethnic/racial family roots, now picture being 100% that and some one calling you a derogatory name.
As far as I know it is alway one letter abbreviation of Japanese. JRPG, JPop, JDevelopers, JPublishers, JCharts. I think those are the one's I've seen, that are appropriate.
Americans we tend to be a little more flexible, and don't care what people call us... unless it involves are race, religion, ethnic group, color, creed, a second nationality, physical or mental disability (don't call it a handicap), political beliefs, financial background, or sexual orientation. Other than that it's cool, you can say what ever you want.
I would cite regulation, but I know you will simply ignore it.
