I have worked with both types of managers and many in between. My perception is that the sandbagger tends to be more accurate and executives tend to respect them more. The problem for the sandbagger comes when the project is large enough to get multiple estimates and the sandbagger gets put on the spot to explain his estimate. Our current project was estimated by our former VP at 4X and an outside consultant estimated the project at X. The board called the VP in to explain why his estimate was so high. We are now well into the project and former-vp's estimate is reality and former-consultant is gone.