I use both rangefinders and slrs and have never understood why people say this. You always have to do this, with either type of camera. The moment of exposure means that you have already pushed the button. Seeing it or not seeing it makes no difference at that point. No mirror movement means slightly less mechenical lag between pushing the button and the shutter opening. It's a very small amount of time. I think you would be hard pressed to show an example of when it was really significant.
Originally Posted by Gerald C Koch