As a matter of habit, I usually create Variables with names starting with underscores as CONSTS - so for example, I'll create a Variable called "_BaseFolder" for the path of the folder which I want to watch for files.

I've found a problem when I want to run an Add/Edit Task of type 'Calculate Variable'. Variables starting with underscores (and maybe other non-alphanumeric characters too) don't appear in the Variable Pulldowns.