I would agree with your first statement "good for learning" if learning would be about maximing each result of a learning experience. But actually learning is more efficient if you do mistakes that you can evaluate and make decisions based on them. And if you don't say a number you don't make a mistake. Therefore I would say that not saying anything may increase your knowledge of your value but it doesn't increase your skill. Saying the wrong number, walking out happily and later learning from a coworker that you earn only 25% of him, then you learn something and look for another job.