Candor is a good thing
Our business cultiure has long been caught up in maintaining the illusion of omnipotence, omnicience, and infallibility, but there are no real live human beings who actually have any one of these qualities, so it's not at all logical to pretend to that one does; it is even, in economic terms, inefficient.
So I have long maintained that those who don't know the answer to a question should say so and then commit to finding out the answer (but definitely do your homework before an important meeting). Likewise, supervisors who do not have the expertise to know have any number of times demanded the impossible from their subordinates; thus it is the duty of any professional worthy of the title when faced with an impossible demand to say so and say why; and if the boss won't listen to reason, then maybe it's time to look for another job anyway (but there might be at least a tiny possibility that what he wants is actually feasible; so be careful). And there are things employees should say "never" about (anything illegal or immoral); though it's very foolish to assume or state that a particular situation can never occur.
That said, tact and an air of reasoned confidence are important in all work situations, especially when dealing those who do not know you well.