Whether or not the truth should be told is relative to the person being told and the nature of the topic.
As Ruchira said, to be at peace with oneself, say the truth. This essentially should be the case at all times, but if breaking truthful yet hurtful news to a person will cause disturbance in said peace, it may not be best to say it immediately.
Being blunt usually doesn't help, but sugar coating words doesn't change what they mean. If a person you're telling the truth to is easily angered, unwilling to listen or generally stubborn, you can expect a negative reaction almost regardless of the nature of you're truth.
However, many and what I'd like to say most would prefer being told the truth as it enforces trust within those conversing.
I think you should assume the truth will be discovered, and determine if you're the best person to find it out from.