Gotta go with Sis on this...It Depends.
First I guess it would depend on how well I know someone. I don't really mind saying something that is not favorable...but I have to see some upside in it. Are they really being serious that they want to know the truth? If they know the truth is it going to make a positive difference? Is my answer more likely to harm them or me? If it is only me I can be more honest.
Its the inbetween people who are the most difficult to deal with on a tough question. Someone you barely know is probably not worth the effort of getting too worked up about being too truthful with a tough question. Be truthful and let the chips fall where they may...or keep your mouth shut. Who cares? Someone you know really well is probably more able to accept a tough answer. They already know the depth of your affection. But the in between peson is a bit tougher. They may not be as comfortable with my answer...and I have no intent to harm. But being false is not good either. So, it is a crap shoot...it just depends on the question or the answer.
Like most things in life...There are no simple answers.
Whatever prompts you to ask such a question? :D