It depends on the format of the broadcast, the size of your TV screen, and the distance you watch from.

I watch a 27" tv from about 10 feet away. If the show I am watching is broadcast in Widescreen I watch it in Widescreen mode. If the show is broadcast in 4x3 then I watch it in Full mode.

If your screen is smaller or your viewing distance is further you may want to use full as the image may be too small to comfortably watch it.

I never use Zoom.

Thanks. I left it on Letterbox last night and the channel I was watching looked the same as before, except the picture was clearer and it had a narrow black section at the top and bottom of the screen. It's a small tv (maybe about 17" diagonally) and I don't have any idea how the stations are broadcast. I'll try Full tonight to see what it looks like.