There's a big difference between units/characters in the game using those words and us (modern people talking on a forum) using those words. I'd say that the first is necessary for an accurate representation of the time period whereas the second is just people being rude.
That said, it is a bit odd to enforce what appears to be an Australian/New Zealand/Canadian/American thing on a global forum.
Hmmm I'm very surprised at the results of the poll! Nip is most definitely a racial slur and was used to garner hatred against the Japanese during world war II. Most western countries considered the Japanese to be small, weak and not suitable for soldiering. The British didn't think the Japanese could take Singapore and severely underestimated the Japanese soldiers capabilities. Going so far as to say that perhaps the Japanese could fight Chinese soldiers who were just as terrible as the Japanese but against the might of the British Empire they have no chance.... Well look how Singapore turned out. The United States were shocked when it came down to fighting island to island against nearly always fanatical enemies.
The earliest occurrence of the ethnic slur was probably in the Time magazine of 5 January 1942. The American, British, and Australian entry of the Pacific Ocean theatre of World War II heightened the use of racial slurs against the Japanese, such as jap and nip.
Generals in the south west pacific went so far as to say Nips are just vermin for exterminating.
In fact most of the terms that the Allies used were for the very purpose of instilling hatred of the enemy in the troops, including the term Kraut (a reference to sauerkraut or them sauerkraut eating Germans). It was particularly favoured by the Americans, the British preferred the term Jerry or sometimes Fritz.
That's what a little research has shown me.