At what mileage interval should this procedure be carried out? Does it help to make for smoother shifts, or is it just preventative maintenance to stop it breaking in the future?

My car shifts smoothly if I accelerate slowly, but on any kind of throttle that requires the car to move a little briskly in the lower gears, the shift can be very quick and leave me with a feeling that it slipped/jerked into the next gear, rather than shifting in a controlled motion.