tl;dr:(1) The work still takes the same amount of time regardless of the accuracy of your estimate. (2) No matter what you do, estimates can never be fully trusted. (3) Imposing estimates on others is a recipe for disaster. (4) Estimates become more reliable closer to the completion of the project. This is also when they are the least useful. (5) The more you worry about your estimates, the more certain you can be that you have bigger things you should be worrying about instead. And more.