Do you always tell the truth?
I don't mean, when your wife/gf/sister/mum/friends asks "Does my bum look big in these Lycra hot pants?"
Clearly, you should lie like a statesman under those circumstances. Do not say, "It makes your already fat arse look like two VolksWagons... fighting over a parking space... in a bag."
Unless life has been really good to you recently and you are in the mood for a change of fortunes.
When you are collaborating with other musicians... do you always tell the truth about what you think of what they are playing?
Again, I not suggesting you should tell the person who played the bass part on your new recording that it sounds like shit. Unless you have already found a new bass player. You can still be tactful. (Unless he's a truly terrible bass player.)
A little story.
I was working on a new track with my band recently and I discovered a great chord. I found it in the usually way; by mistake, and the more I played it the more I liked it.
At a rehearsal I introduced my discovery to the band and the guitarist, always one to speak his mind, told me quite concisely why it was wrong. I was a little hurt at the time, but we worked through it and found an even better chord that was just as entertaining but which didn't sound like it belonged in a completely different song. I quite like our new chord now and probably use it too often.
The moral of this story is; when working with other musicians, don't be rude or offensive, but do tell the truth in a constructive way. That way everyone benefits.