There are two sides to this argument of whether the ground lug should be up or down.
Most electricians will tell you to have the ground lug facing up to prevent them from shorting out.
If the lug is up, you have protection for a loose plug that continually pops out. Something that falls on the plug such as metal is not going to short out the receptacle. It will hit the ground lug first.
If the lug is facing down and the same thing happens, the receptacle can short out. On the other hand, placing the ground lug facing down will not cause the plug to pop out or become loose.
Depending on the area you are placing the receptacle in and what it will be used for is another determination of which way to face the ground lug. It is never a good idea to face the ground lug down if you are using the receptacle for operating appliances or other large pieces of equipment.
You should use common sense. The best thing to do is call a professional, licensed, insured electrician to install the receptacle the safest way for your individual situation.