[jfarmer]

Those are two separate questions, IMO. In the context of a job interview, however, I believe giving the interviewee permission to say "I don't know" makes the interview go better because they'll spend more time focusing on the interview and less time trying to figure out "what the interviewer wants."

I usually give a little preamble telling them to feel comfortable blurting out nonsense. I'd rather have them blurt out nonsense than sit there for 30 seconds in silence because they're afraid to say something stupid.

You can see the interviewee relax almost immediately.