Never. Don't even think about it. You'll be in a bad fight if you do. It's considered a horrible insult. Strangely enough, some black rappers use the word, but it is considered a word that blacks, especially urban blacks may use among themselves, but nobody else. Even then, it is considered bad by many other blacks.
Also, 'negro' is technically accurate, but seldom used any more. Use either 'African-American' or black.