Mustafagander--
Actually, it is the "parallel" yaw dampers that should be switched OFF for takeoff and landings, series remain ON at all times (at least on the B707). Hibred types, like the L1011, series for all flight ops except at A/L track where it switches to parallel for runway alignment. Recently delivered a TriStar on its last flight before being parted-out. Even though it had not had a gold-wire check in years, the Approach/Land
function worked perfectly with centerline displacement of no more than about six feet.