I'm sure you have heard to always get three estimates for any work you are going to have done. Well, that depends on a lot of factors. If a contractor has been recommended by a friend, you have seen his work and are satisfied with the quality, and YOU have met and talked with the contractor and are comfortable with him, then maybe you only need to get his estimate. If you feel his price is reasonable, I would say go for it. On the other hand, if you are getting bids from contractors that you have no personal experience with, get three bids. If the bids are within 10-15% of each other, then choose based on your comfort level with the contractor. If the three bids are really different, more investigation needs to be done. Either, a contractor forgot something and the bids are not "apples to apples" bids, or there might be something suspicious about a contractor with a "out of range" bid. Look closely, buyer beware.