Beat your estimate and you are a hero, meet the estimate (through bad luck or unforeseen disaster) and you are a valued member of the team who delivers on time.
This is always a good standpoint. Where I work we call it underpromising and overdelivering. And it works out just like stated above. It's all too true that people will hold you to the first number out of your mouth. Especially if they dont like it. Beating that number will turn anyone's opinion upside down.
I am Takophiliac. The evil souls remind me and in the darkness find me.