Under-promise, over-deliver. This idea can apply to several different areas of life, but it's a great managerial mantra. Do you want to be the kind of person who has wildly optimistic goals that they never meet, or do you want to be the kind of person who sets measured goals and ends up exceeding them by leaps and bounds? Although this is about image, image is extremely important.
Don't be the kind of person who never shoots for the moon. Staying measured in your goals doesn't mean that you should always play it conservative, never setting high goals. A manager who never punches above her weight can come across as lacking ambition. Even the conservative poker player knows they need to go 'all-in' from time to time.