Sometimes we need to be tactful and sometimes we need to keep our mouths shut. An anonymous commentor on this site, responding to a blog about secret-keeping, makes an excellent point about aggressive truth-telling and sent quotes from William Blake, Alexander Pope, and Alfred Adler.
Listen to Blake,
"A truth that's told with bad intent
Beats all the lies you can invent".
And Alexander Pope:
Tis not enough your counsel still be true;
Blunt truths more mischief than nice falsehoods do"
And psychologist Alfred Adler:
"The truth is an awful weapon of aggression.
It is possible to lie, and even to murder, with the truth."
A rule of thumb is to be absolutely sure you aren't lying to yourself before you decide to tell the "truth" to some poor deluded soul. Since it is difficult--if not impossible logically--to know whether you are lying to yourself this rule would go a long way to restrain bluntness--maybe too far. We might never be able to speak again! Another rule might to consider whether your timing is good. If you don't just spit it out because you feel the need to, you may not spit it out at all.