This rule applies to button up shirting, and in its entirety really reads, "Never button the top button unless you're wearing a tie." The idea is that if you're not wearing a tie, there's no reason to close the top button—a move that may be associated with nerdy guys or the conservative clergy. However, rockstar style legends like The Who and the Pet Shop Boys started buttoning their top button as way of looking different from their peers and parents way back when. Alternative director David Lynch has been wearing the look for more than 20 years.
As many outdated rules go, this has origins for how to dress in the boardroom, a place that typically—but not always—lacks style or creativity. Deciding whether or not to break this rule should not be about committing some sartorial sin, but rather about what's comfortable. A large reason why guys choose to keep the top button of their shirts unbuttoned really comes down to the fact that they find it physically uncomfortable. Whatever you choose, don't get this twisted up with the Chicano movement of Southern L.A., as that consists of buttoning only the top button, and that's something you should avoid unless you're about that life or enjoy being called a poseur.