Replacing single pole light switch with dimmer
If it were me, I'd take all the grounding wires out from under the screws.  Then I'd collect up all the grounding wires, including the one from the new dimmer, and an extra bit (6" or so) of spare bare wire of the same size.  Using a twist-on wire connector, I'd connect all the grounds together.  Finally I'd slip the other end of the short bit of wire (pigtail), under one of the screws in the back of the box.  

This way all the grounds are bonded, and you're not relying on the metal box as a fault current path.