One must know when to extend the truth and when to tell a white lie. I believe there is a fine difference between being truthful and being painfully blunt. While, yes, you are still being truthful when you're blunt, there are always more tasteful and tactical manners than being a flat out arse. You being overly truthful and mean on top of it isn't going to make what you're saying any better for the other person. On the other hand, if you were to be an arse and blunt to a nemesis who genuinely deserved it, by all means, go ahead.
With that being said, I personally keep my silence until called out, when it comes to things I don't want to talk about.