Great point. The times I've pretended usually come back to bite me and are eventually more embarrassing than just saying "I don't know". Now, when I'm tempted to pretend, I think about the embarrassment that will probably result, which gives me the courage to say "I don't know." On a related note, I was once in a meeting with a couple brilliant NASA scientists, and was surprised how often they admitted they did not know something. There were a couple of the smartest guys I've ever met, and the had no qualms about admitting things they didn't know. That helped me to start saying "I don't know" more often as well.

I'm a big proponent of "I don't know" in the work place. People universally use "I'm not sure" in its place. It's mostly harmless until you have a conversation like this:

Question: "Is it A or B?"

Answer: "I'm not sure, but I think it's A. But I'm not sure. It could be B. But I think it's A."

It's annoying when that answer is seen as better than a simple "I don't know".

Be careful, though. Replying "I don't know," or "What's that?" can have the effect of calling someone's bluff. If they are someone you do not want to offend, the situation can get awkward quickly.

Usually if it's something I have only very basic knowledge of, that's what I'll say. It's not always saying "I don't know" if you really mean "I'm not certain".

It's more about being honest about your level of knowledge. Sometimes that means grey are or saying "I don't know, but I will find out." OR "I don't know, but let's get a hold of someone that does."

That would happen in more of a professional environment than a conversational one :o)