Aesthetically, do what pleases you.
Aerodynamically, you'll be catching slightly more air. I thought about opening up the bottom slot but realized that opening it up won't serve any purpose unless you get an oil cooler. The radiator starts from the upper slot.
Also, the area is metal so you do run the chance of getting rust spots if you aren't careful in repainting the cut areas. If you ever to want to sell it, you won't be able to return it stock very easily either.
With salty winters here, opening up the bottom slot only can lead to rock chips and eventually rust, so I figured I would leave it alone.
I'll be lucking if I can escape this winter without any broken windshields.
