Speaking with a friend the other day, he paid a compliment to his coworker by saying: "He's a great worker, never says no to anything!" This line was supposed to praise the person's work ethic; their ability to get things done.
I believe in strong work ethic and getting things done too, but to praise someone on their inability to say "no" is dangerous. Going the extra mile and bending over backwards to achieve a goal or please a customer is one thing, and efforts like those are great for an organization to achieve. But when applied to an individual, it leads to burn out.
Praise your coworkers and employees for working hard, but also recognize when they wisely say no. Would saying yes be at the detriment of their health? Their family? Then it's time to say no, and make sure you praise that also.