Here's the thing, shop floors, whether wood or concrete, are rarely perfectly flat. I used to make one leg purposely short, then weld a big nut on the bottom, with a short bolt and a big pad. That way, you can turn the bolt up / down, just like on a washing machine, etc, to get it to sit "square."
On more important stuff like lathe/ mill tables, you need a way to level all 4 legs, or at least 3
