I have always done it just so gravity doesnt take over, after time the pluging and unpluging the connection gets some play in it. But, I would also like to know if its a code requirement, I checked some and did not find anything. IMHO its a good practice....
Without knowing what type of equipment is being installed, its hard to say.
If you can be a little more specific that would help.
EDIT: I have looked several Times in the 2008 code and cant find any mention of twist lock receptacles. I think its just good form to do it even though not required but dont quote me on that.