I think that saying "no" for a certain kind of people. Some people I know are masters of saying no, even to the most trivial requests which they may satisfy without compromising their time or resources.
It is a must, though, to learn to say no. That will not make us bad people, just more organized and responsible (with others and with ourselves). Some people say yes to everythings and then they cannot deliver.
Saying no to ourselves can be more tricky. We tend to indulge in distractions that are supposed to be impotant or act as rewards for a job well done or to compensate for too much work, but it is easy to lose perspective and procrastinate on our priorities every time we deviate our attention to pay attention to the constant whims our minds bring to the table.