At Slate, here's a good article looking at the filmic/television-ic history of buttoning up that last, top button. There are several uses of this character shortcut. It can represent a slow-guy like the classic examples of Billy Bob Thornton in Sling Blade and Forest Gump. Sean Penn in I Am Sam buttons his shirt all the way when he gets a job at Starbucks. Urkel wasn't at all slow so his top-button represents a social deficiency, rather than an intellectual one. And most recently, Monk does the same; in this case, it's a symptom of OCD. In fact, this is the strongest argument for top-button=something f'd in the head, since he only buttoned up, sans tie, after he got all cussed-up after his wife died.
Don't button your top-button. And don't tuck in t-shirts either.
What if the t-shirt is underneath a dress shirt, and you're wearing a sweater over both?