I read a lot of the past threads here on how to level a mill, and it always seems to boil down to personal preference on some of the details. ForrestAddy used to say you could practically bolt a mill vertically to wall and it would work fine. Seems the only reason to have them level is convenience of using a machinist level for work set up, as mentioned above. Obviously you want all four corners on the floor so it doesn't rock.
There's also a debate as to whether pads reduce vibration or make it worse, as AdamC mentions. I've not seen anyone post any tests on it. Some softer materials seem to magnify vibration (like a wooden basketball court vs. concrete floor). On the other hand, vehicle engine mounts typically ride on rubber type grommets to reduce vibration.
I leveled my mill using steel shims with a single layer of inner tube rubber underneath.