As of 2013
[update], the probe was moving with a relative velocity to the Sun of about 17 km/s.
[12] With the velocity the probe is currently maintaining, Voyager 1 is traveling at about 325 million miles per year (520 million kilometres)
you don't even need to worry about 10's or 100's of km of accuracy, its travelling almost 20km every second
you are forgetting that the further out you go, the less the solar gravitational effects will be ( inverse square law = every time the distance is doubled, the field strength is only 1/4 as strong)
EDIT: just read your last sentence properly, Bob, that may be what you were referring to ?
The gravitational effect isn't just reducing a bit, its reducing quite substantially
Dave