An important aspect of Requirements Management is managing expectations.
One of the basic rules that applies there is: Underpromise and Overdeliver .
Under promising means that you say Friday when you assume you can deliver on Wednesday. Always build in an extra percentage.
Over delivering means that you deliver early, on budget and better than promised.
This rule is mentioned in the Requirements Management with Use Cases course I give.
Richard Templar wrote about it in his book Rules of Work, The: The Unspoken Truth About Getting Ahead in Business