I'm not a pizza expert by any stretch, and Des Moines certainly isn't a hub of food innovation, well except for the Blue Ribbon Bacon Festival .I would assume triangle is traditional. I think the square slices are mostly just a Midwest thing and I consider it to be a form of trolling to slice a pizza like that anyway.
The real Italian restaurants around here seem to mostly do the square cuts, and most everyone else does the triangle ones. All tastes the same no matter how you slice it!