I always try to be tactful, and personally dislike it when people are blunt. I think it is inconsiderate and is basically the same as being tactless. Being blunt can often hurt people and should only be resorted to when absolutely necessary. Other than when it is needed, why needlessly hurt others?
The Bible says to speak the truth in love, so say what needs to be said, but be kind about it!
It is always possible to be tactful. Even when you are dealing with crude & inept individuals, you should strive to take the high road. You never lower yourself to their level!
I personally find bluntness much more important, but I recognize that tactness is sometimes the best way to handle a situation.
For gosh sakes just get it over with. They'll get the same message anyhow, but just think "Oh they're considerate." -INTJ