Presumably you're using those two questions to determine whether you should say aloud a thought in your head. The problem is that "is it useful?" is pretty much semantically the same as "should I say it?"
There are useful statements that should be avoided.
Some statements are true but not useful. Like telling your grandmother that she looks like an old hag. Some statements are useful but not true. Like pretending to be sick to avoid school. The damage from the latter can be more subtle but runs deeper.
Things that are both useful -and- true, those are worth saying.
It isn't always easy to tell what is useful, so it's best to rely on your intention. It's good enough to mean the best. When uncertain about usefulness, err on the side of being truthful.
I think in the modern business climate there is an emotion around wanting to do the right thing, and it could be something of a appealing idea to latch on to always being truthful, or radically honest. However, I think this is an easy way out, for several reasons, most notably that honesty is not the same as fact, and I would say facts are much more valuable than your honesty, and ideally have nothing to do with you personally.
When talking about opinions I think many people can't help but be absolutely honest when absolutely necessary, too, and this kind of honesty is different than say "tough love" or even at times "straightforwardness." This is the continuum of communication, I'm trying to speak to, and perhaps radical honesty is a part of it. But if everyone were radically honest or straight forward I just simply have to lie or be subtle or sarcastic or cynical or just be silent to get any attention, or at least that's how I feel I might react to it.
> On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.
-is it true?
-is it useful?
If the answer is no to either question, just skip it