Does the screw look like this below. Otherwise I would not unscrew it. When looking at the EP350 guide just now it does not say anything about a ground screw being present.

http://www.axiomaudio.com/troubleshooting_ep500.html#

This is what the one on the EP500 looks like.
click on view chasis ground screw

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne