There are a couple of questions to ask yourself before actually naming a variable:
A note on global values. Global values can also be defined when actually naming your variable. These values, which remain identical across an entire site need to be identified at the start. This can be something like a list of country codes or language codes. An example of the use of global values to validate data entry is illustrated in the variables behaviour page.
Thus, defining global values before getting started is a very healthy thing.