Nulling is done do reduce the handlecount and allow the game to reuse the same memory space. With globals variables it is mostly pointless, because they already have a predetermined memory space, so they can be overwritten without problems(except handles).
The problems with handles(such as locations) is that the variables don't really contain the values. Instead they point to the value somewhere else in memory. Due to this, nulling or overwriting a location doesn't really help, as the actual value will still be somewhere in memory.
OT: Remove the location after you have finished using it. This pretty much equals to saying that "Alright game, I don't need this location anymore. Clear it up, okay?"