I never actually thought about it until this thread, but I think just because of how I was raised or whatever, I put the pizza on either a foil-lined tray or on a sheet of foil. But this morning I put in waffles and realized that for waffles and toast, I put them directly on the rack, LOL.
I think it's because a tray or foil sheet makes it easier to grab the pizza since it's in the middle (bought a bigger toaster oven) and less chance of touching a super hot rack when I reach my hand in, LOL. My toaster oven does come with a pizza stone, which I use for a regular frozen pizza but is too big for something like a personal size frozen pizza or heating up leftover slices for lunch or something.