Using receptacle mounting screws for ground connection
You cannot use the mounting screws as a grounding path. 

However, you can use hard metal-metal contact between the receptacle yoke and the grounded steel box.  For this, the yoke mounting screws have to bottom all the way down hard.   Here's what doesn't work: 


the yoke doesn't bottom all the way because its ears are catching the edge of drywall or finish, which is the normal way receptacles are installed
there is paint on the box or yoke (make sure to mask it before rolling the walls!) 
most receptacles come with the mounting screws "held captive" on the yoke by those little squares of paper/plastic. Those are insulators and prevent good grounding. 


Boxes or yokes with special spring contacts are only good if listed as such: i.e. their instructions specifically say that's OK. 

It doesn't matter if the grounding path "meggers out" OK at the moment.  That could change over time, or as someone is pushing a plug in or pulling it out.