Well, it depends what kind of development you're doing. If you're doing plugin development, you need to use the functions because you'll never know for sure what the url structure will be. If you're doing specific work on a specific website, it's technically ok to hard code it, but that is still inadvisable, since it precludes the possibility that the url structure could ever change.
Whenever I develop plugins or themes, I structure the program in a class; that way, I can save repetitive values as properties of the object (e.g., WP URL and template_url). That way I can use them quickly without calling the functions repeatedly.