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)
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".