simply put, if you will need to use the throttle to adjust something soon, keep your hand upon it.
on takeoff until you reach V1 (transports).
on takeoff to make sure the throttle does not creep back in a light plane and there is no doubt you will continue the takeoff. (no need to abort/reject).
holding your hand on the throttle in really rough air on takeoff MIGHT make you grab something and pull it the wrong way, so use caution.
on landing approach, your hand really needs to be on the throttle at all times, esp in rough air (see above for caution though) as you may need to add power in a big hurry.
You should always be able to fly the plane with one hand on the wheel one on the throttle, except in extreme circumstance.