IMO, steering centering should never be adjusted for road grade. If your steering is slightly to the left due to a right grade, there's a reason for the steering wheel not being straight. Then if you are on the highway in the left lane and the road is sloped left, your steering would be slightly to the right. As it should be. Assuming all roads grade to the right is a bad assumption. There are plenty of roads around here that slope down to the left: it depends on where you have drainage in that area.
Mike
Mike