The default, if not specified is always None (NULL). Some DBAs will argue for and against storing NULL values in a database, but I personally use NULLs simply because they allow more flexibility. For example, say you have a text field in one of your tables. This text field should be set by the user, even if the text is empty, the user should set it. This would be stored as '' (empty string) in the database. So now, if you wanted to go through the database to find records where this text field as never been set, all you have to do is search for None.
That may not be the best example, but you may find lots of cases where you can use None (NULL) to your advantage. A better example might be for a time clock. You have a start_date and end_date. If end_date is None, then you know that person hasn't clocked out yet. It's all a matter of personal preference.
