to really get things level, sometimes you've got to use your pick. at some point as you go down you'll run unto soil that just won't go lower, pick it and it'll go down and can then be leveled with the hoe. if you need to raise to fill a little hole, it'll often go higher, but the level action will bring the mound down to where you need it.
also swing the camera angle down to the ground.
another trick is to do a "sideways" pass on your road... so if you're on a slope don't level up or down hill, but at the same level as you're standing, then move up/down the road a bit and repeat.
i like to level wider roads b4 adding cobble, then do a pass with the cultivator to fill in with grass. you can even get some cool effects cultivating the soil b4 adding cobble. but overall i still find that after i get a nice path made on a hill, cobbling it messes it up some. cobbling sideways or doing a sideways pass or leveling from the high side if i just did the low side or vice versa often helps keep things from getting too out of whack.