The correct installation of a receptacle in a grounded metal box is to break off all four ears leaving only the end tabs. If shimming is necessary to space the receptacle out, then the broken off ears are placed around the screws between the tabs and the box. This establishes an adequate and approved conducting path. Just the screws and threads would test OK but are not considered adequate for high currents.

**EDIT**

Note that any **paper screw keeper must be removed** on at least on one end. This allows one end tab to make face-to-face contact with the metal box.

Good quality receptacles have a paper screw keeper on only one end and it does not have to be removed. The tab on the other end has a metal wire keeper which allows the tab to make face-to-face contact with a metal box as the screw is tightened. If the box is too far back, then use the broken off tabs as conducting shims.