Vic - the ball is indeed very sensitive, and you'll see slight deflection in the ball before you see the corresponding roll effect on the attitude indicator, as you describe. If you're centered on the air but not on the ground, then either what you think is level on the ground isn't (tire pressure differential, the ground isn't perfectly level). If it's the other way, you might be a smidge out of rig or trim - something's causing you to fly with one wing very slightly low. Regardless, if you shim that tiny amount out, you'll most likely not see adverse affects.