How honest should you be?

On NPR Seattle (stream) right now is a show exploring the danger of white lies. Philosopher Immanuel Kant believed lies, of any size, were wrong. But the NY Times ethicist Randy Cohen thinks that lies can be good in some cases, such as when they protect someone’s life.

So when working with other people, what are the ground rules? How do you decide how honest to be with people about:

Their performance / quality of their work
What you really think of their ideas
What your true motivations are
How you feel about how things are going
The reality of the project schedule

I bet there is a high correlation between how honest the average person on a team is, and how well that team performs.
Anyone agree or disagree? What factors contribute to how honest you are?