Here is my 2c worth...
Pump,
I guess the answer depends on how safe you want to play it! In the approval letter INS will list all the required documents that need to be carried for stamping, and one can ask for any missing ones before stamping. I don\'t think some one at that point of time will ask you some thing like "employment letter" when it is not mentioned in the list of document. On the other side I guess INS can question any one at any time about the validity of GC if they suspect any fraud.
But taking employment letter from your employer (on the name of stamping) may be a good idea. It\'s just another document in your GC file which says (officially) that as per your employer at the time of GC stamp you are still working in that GC job or atleast that GC job is still there. Later because of any reason (may be while applying for citizenship) if there is any question of fraud then this letter could be somewhat helpful.
Good luck
PCee