I agree with the idea that it is situational and it also depends on how the truth is expressed.
Along the lines of "someone's cooking", If asked what I thought, you give constructive criticism if you didn't like it. Point out what you did like about it and what you did not.
As for more personal things, which I am sure is to what you were asking, if asked a direct question, tell the truth. This can also be a "constructive" truth as mentioned above. If someone is doing something that really bothers you, should you tell them? Yes. But don't attack them, do your best not to put them on the defensive and do not "blame" them for how you feel.
Is it hard to tell the truth? Hell yes it can be. Does that mean that you should not. Hell no. You can be worried about their reaction, but always remember, it is better that they find out from you then from someone or someplace else or for you to go on and on in pain and suffering (if that is the case).
If the truth does not come out, nothing can be resolved.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.