With the newest release of Microsoft Office, we need to add few more lines to disable What's new prompt at first launch.
/usr/bin/defaults write /Library/Preferences/com.microsoft.Excel ONWhatsNewShownItemIds -array-add -int 17090
/usr/bin/defaults write /Library/Preferences/com.microsoft.Excel ShowWhatsNewOnLaunch -bool false
/usr/bin/defaults write /Library/Preferences/com.microsoft.Excel OUIWhatsNewShownItemIds -array-add -int 17091
The same for word,powerpoint,onenote,outlook.Hope this helps.
