Originally posted by FissionChips:

That, plus I usually stand on wooden floor pieces snapped together to be certain I'm completely level.

Oh, and if you want smoother gradients as well then holding the shift key levels to the target rather than where you're standing.

Standing on a block levels the ground to the point the ground is at straight below you. In other words, blocks are ignored.
Holding Shift averages the target position. This can cause the exact target point to rise or drop as well.