Being able to say no helps prevent your business from failing and helps maintain a healthy economy for the creative world.
Now I know what you are saying, "What a negative article! I was taught being a yes man/woman is the best way to progress in life!"
I would like to take a moment to disagree with you. I have been designing for a few years now and being only agreeable with every client or person you work with can be toxic. Many people won't take advantage of you and will work with you and whatever terms you set, but in some cases, people will try to squeeze anything and everything they can from you. As an artist or designer you need to be able to identify these people before it is too late.