Absolutely - admitting when you don't know something is indeed wise. But there's a fine line... if the person in question seems to never know anything, then you start to wonder if that air of deep calmness is actually just "nobody's home". By the end of the movie I was getting that impression with Qui-Gon, which I'm sure was not the movie's intent.

I'm not going to go back and count the number of "I don't know"s (that would require me to actually watch TPM again)... I just remember that I was struck by how often he'd point something out that he observed, as though it's important, then when asked what it meant, he'd get a concerned look on his face and say "I don't know". It just seemed like dialogue cop-out.