You'd have to be between stars to not be influenced by other bodies.
Any path in the Solar System is bent by the Sun's gravity. Picture the planets as various size balls rolling around in the bottom of an empty concrete swimming pool.
If you bounce a ping pong ball off one of them, to get it to the other, you will have to follow a curved path to get there. And if you are going uphill, you will have to aim for a point on the target planet's curved path ahead of where it is, because it will take some time, and you will lose momentum.
The variables are many, and the formulae are complex, but everything is known to many degrees of certainty, so it is still rather straightforward in that respect.