"Don't be evil" clearly is bullshit to the extent that you can always rationalize an evil action, and three words aren't going to stop you from doing what you want.<p>On the other hand, though, a mantra like that can help entrench a culture that makes it harder for "evil" ideas to find roots.<p>On balance, I'm not sure a mantra makes a difference.  It's sort of dangerous when it becomes so entrenched that you start to think your actions are <i>by definition</i> non-evil.  From time to time, we do see Google pulling out "it's not illegal if the President does it"-type rationalizations.