Why would you care if there was evidence that you'd logged on? If you did log on, then you have a valid login and password, and therefore you were meant to log on - so why hide it?

Besides, the network admin can set up all kinds of auditing tools to keep close track of who logged on when. You can never know what evidence you're leaving behind. Making the screen look like you were never there is pretty meaningless, unless you're trying to hide the fact that you logged in from a moron.