Re getting a pilot with an emergency to squawk 7700, that's up to the controller, taking into account the documented unit procedures.
If you already know he has an emergency, you don't then need him to set that code on his transponder, unless you want the red alert on your screen.
Other factors come into play, of course, the main one being that if you will be subsequently handing the aircraft off to another controller, the 7700 on the screen will leave no doubt as to the status of the flight.
From a human factors point of view, I would also find it useful to have that alert up, if for no other reason than it lets adjacent sectors/controllers know that you are dealing with something out of the ordinary. Situational awareness.