It looks like only the 30 Amp plus NEMA connectors require ground to be on top. It does appear technically "safer".
I call bullshit on the whole "earth pin position as a safety measure" debate anyway. Copper wiring of any substance is physically to rigid to move, even if completely free of other mechanical constraints, so isn't going to "drop onto the live pins" no matter what, even if the internal arrangement of a socket facilitated it. (They - don't they're insulated, shrouded, separated etc.)
For example you plug in a drill but it slowly works its way out, you are working with a screw driver against the wall and accidentally drop it, at this point the screw driver can come to rest on the live pins of the drill that is plugged partially into the wall. With the ground pins up the screw driver will pick a side and fall off...
Ah, so awesome.
The longer ground pin at the top provides a mechanical advantage when a downward force is applied to the plug. Since this is the most common accidental force (e.g. something dropped or a cord on the ground dragged) it slightly reduces the risk of disconnect.
In addition, the ground at the top allows for a better view of the pin and receptacle hole.
They hold nicely in ground down receptacles though.
The insertion force of the one ground pin is lower than the two live ones. So when force is allied downward the plug pivots out of the receptacle.
In a ground down orientation the plug will pivot into the wall. Since the ground pin moves easier than the other two, it moves into the wall instead of out.