According to the discussion here, it is always safe to cast from Graphics to Graphics2D. However I am not able to quickly find the official Sun statement on this.
The reason it is valid to cast from
Graphics to Graphics2D, is because Sun
have said that all Graphics objects
returned by the API in Java 1.2 or
above will be a subclass of
Graphics2D.
Another hint here with the same conclusion.
Graphics Object can always be cast
Graphics2D g2d = (Graphics2D)g;