I disagree. You do not let an idea brew in your mind in order to improve it. You work on it. You give it form and then improve it. You don't improve your ability to draw by envisioning the picture in your head for longer than necessary, the same is true of writing.

Secretum knows it well. The point of NaNoWriMo is to simply create. She wrote everything that I was too lazy to write. (Maybe that's why I only made it to 11,000 when I tried NaNo?) Creating great work doesn't happen right off the bat, every word is subject to revision the same way that every pencil stroke can be erased -- and will. It's all about creating, creating, creating, working, working, working, and less thinking about it! Letting anything sit in your head will never improve it. Giving it form is the only way.