The washers go on the inside of the metal clad box, then secured down tightly with a metal bush using a bush-king spanner. If it is a painted metal clad box for example, the bush on it's own will not give you any earth at all. the serrated edges of the washer dig into the paint and provide the linking earth. You wouldn't need to earth each box from the socket if you did this, just the first box.
Of course, for good practice it is good to earth each box, and that is how it should be done IMO(but doesn't have to be). I like to earth each box individually as if an earth cable comes out of the first/second/third socket, the rest of the boxes will not be earthed. Doing both ways is a a bit of a "fail safe". If one way doesn't work, at least the current will pass through the boxes and find the nearest earth.