If there's enough loose, porous fill underneath, you will get just what you describe and everything's fine. In fact, for a small shed it's probably *better* than digging post holes. Posts in the ground can be lifted by freeze and next thing you know, your sheds un-level.
If water freezes in a loose fill pad and raises the shed a bit, it falls back down after the thaw.
[Mike, ignore any email reply you may get from me. I hit the wrong button.]