The Voyagers made a grand tour using the gravity assist of each planet. But Jupiter, with a 12 year orbital period, doesn't always line up to be useful for spacecrafts destined to further planets. Venus is much quicker on its feet, but light.
What have been the altitudes of spacecrafts' perijove and perikrition (a look up word for periapsis at Venus) to date? What kind of delta-V change could one achieve by passing by Venus as close as possible, on the way to say Saturn or Uranus? Could aerobraking at Venus be helpful in optimizing the velocity vector?