NPR's All Things Considered asks whether it's better to cut a sandwich diagonally or lengthwise—and comes down very firmly on the "diagonal" side. Why? A longer crustless surface area; the narrow corners; simple aesthetics.
The diagonal cut exposes more of the interior of the sandwich, and by exposing the interior, it engages more of your senses before you take the first bite.
I must say that I'm also partial to the diagonal cut. (My mother always cut my lunchbox sandwiches in half lengthwise; I remember yearning to be old enough to make my own sandwiches just so I could cut them into triangles.) But there have to be square-sided sandwichers out there. How do you cut your sandwich?