Best way to store custom values in Drupal 7?
No, there's no need to create your own table for that. Your use case is a perfect example of what should be stored in the variables table (in my opinion).

Since the data isn't particularly volatile, and you only have a couple of values, I can't see any benefit to adding a custom table for it.

If you start finding yourself storing a lot of data in variables, for example a single different variable for each node of a certain content type, that's when you need to start looking at a custom table.