Employment is not a direct requirement for the N-400. But if you have an employer-sponsored green card, employment after green card approval is a requirement of the green card. And if you fail to meet the requirements of the green card, your citizenship can be denied and your green card revoked. So in that sense, the employment can be an indirect requirement for the N-400.
So then the question is, how long do you need to be employed with the sponsor after GC approval? There is no specific law or court case on this, so people have speculated anywhere from 1 day to 1 month to 6 months to 1 year. It seems the most common answer given by attorneys is 6 months (for example see
http://forums.immigration.com/showthread.php?t=154533). Some people on this forum have been harassed in the interview for leaving in less than 1 month after GC approval, but they still got approved after the supervisor got involved.
Once you have worked long enough (whatever that is) to satisfy the requirement of the green card, after that it doesn't matter where you work or if you work. You just have to be prepared to provide an explanation of how you supported yourself during extended periods of unemployment (like over 1 year), with proof if requested (e.g. bank statements, spouse's W-2, severance package, etc.) otherwise they may suspect you were deriving income through criminal means or working for cash without paying taxes.
