This is one of my devils, I don't think you need to be truthful all the time. In fact some situations actually call for lying...
I've been thinking about this phrase and the idea of truth in general. We are taught from a young age to tell the truth and that doing so is a good and Nobel thing.
Then we grow up and realise that telling the truth all the time, is not the best course of action. For instance, telling your mum she looks awful in that dress, may be fine when you are 6 years old (if a bit cheeky), but at 25 it would just be mean.
This, I guess, is tact. Knowing when to tell the truth or not. Omitting particular details and waiting for the right moment before speaking.It is not something you can teach or learn you either have tact or you don't.
I really believe little white lies never hurt anyone. I mean what is so good about being truthful if someone gets hurt by it?
Equally I would argue that one must tell the truth when it comes to the big things in life, i.e don't lie about who you are as this is one thing you'll never be able to hide forever (Of course don't lie to cover up crimes - I'm not justifying that).
Even then there is always more then one side to every story and each is true from different prospectives.
After all if everything was as simple as right and wrong, truth and lies wouldn't the world be a much easier place to live in. Really isn't it all just shades of grey...