Saying ”I Don’t Know”

Originally, this topic came to mind because I see people not using it enough. Meaning they would pretend to know more than they actually did.

Then I was made aware of the flip side, that people also use it as a dead end. Someone may ask you something you do know about and say, "I don't know," as a means to not engage further. 

How often do you use the phrase, "I don't know?" How do you use it?