Putting in the time, not to mention being punctual, shows you're dedicated and serious about this opportunity. You can't be strolling in midway through the day without a valid reason—and no, being hungover or sleeping in is not a "valid reason." Emergencies happen, sure, and those can be determined along the way, and there may be slow days in the office. But no one knows when something drastic is about to go down, and all hands are required on deck. This industry is fairly fast-paced, and you'll find that things need to be done at a certain time, on the spot. That can't happen, though, if you're not there.