Ask yourselves: "how often are we late with deliveries?" Honestly now!.
One of the most important aspects of the estimating process is not just for working out how much you can do in a sprint, it is also about predictability of attaining the desired points rating for the sprint. I highly stress to Agile Central users that they make sure that they have a minimum number of stories in a sprint - and here I recommend at least 10, more if possible. If they say that they can only fit three 13 point stories into a sprint, I point out how predictable are they if they fail to deliver on one story. So, taking those three stories and breaking them down further to become twelve 3point stories means more work up front, but better predictability.
The important bit to remember in all this is that the team is not working in isolation - it is part of the bigger team that includes the Portfolio management team. Development team 'roughly right' predictability and stability is crucial for 'roughly right' predictability at the portfolio level. Which means, in the long run, that your organisation can reliably predict the roadmap you promise to your customers.
As a wise one once said to me: "Take the time it takes, so it takes less time." It seems a trite statement, but it encompasses the basics from Lean. Look at the whole process from start to finish, not just your bit.