Which shelf. depends of Temperature/Raw foods/Size/Humidity
It depends on a few things. My refrigerator, a MAYTAG-(WHIRLPOOL) side by side, is colder on the top shelf at the right side closest to the freezer. So I need to keep things at the top that need to be more cold. So temperature is a factor. Some things like sour cream will actually freeze at the top. But the middle is perfect for them. The milk loves the cold, it stays fresh days longer on the top shelf.
Then Raw foods have bacterias that you don't want to spread. I always cover my raw uncooked meat foods and bag them, then put them either in a pyrex dish, or if they are too big on a cookie pan. You don't want defrosting meats or fowl or fish to leave uncooked drippings (possibly bacteria-laden) anywhere in the fridge or freezer.
You can clean up with baking soda, and a good soap. If it was really a scary dripping I will finish with antibacterial wipes after a good washing.
Then there is size. A large pepsi bottle will only fit in a couple of places. A large cookie pan has a shape to accommodate, etc.
So no set rules other than common sense and safety.
There are usually drawers with temperature or "humidity" controls. I find that I can leave the humidity slots half open, and wrap things like lettuce in paper towels,then in the bags. The GREEN BAGS work really well on many items. Vegies last longer. Some still are wet inside, and if I wrap a paper towel around them and then place in the green bags, they last longer, like bell peppers or tomatoes.