Question: Imagine I have a generation ship that is heading to a nearby star system say 10 light years away, the average lifespan for the crew is 150 years on Earth and is expected to increase by about 5 years with each generation. No cryonic suspension due to the ban on any form of suspended animation on human by law and many policies are in place to ensure the population demography on board is healthy across every generation, so we can now laser focus on the economic and science aspects of this one way trip.

I believe delta v is important when planning trips within the solar system since as the name suggests it is the changes in velocity when jumping between orbits, what about jumping between star systems? Are we going to factor delta v for all interstellar flights regardless? if so why?

Answer: 
Physics is physics everywhere.

1. Your generation ship is coasting between start and arrival point: this means that it will be slowed down or accelerated by the attraction of the body whose Hill's sphere it's passing in at any moment. This influences the delta v. See the plot of the heliocentric velocity of [Voyager 2](https://en.wikipedia.org/wiki/Voyager_2) for a reference, and notice how it keeps decreasing, albeit slowly
[![enter image description here](https://i.stack.imgur.com/GpYHj.png)](https://i.stack.imgur.com/GpYHj.png)
2. Assuming your generation ship wants to land on a planet or orbit it and not just smack on it at several tens of km/s, it will need to slow down during the approach. Once again, this is delta v.
Yes, it's relevant. For a few reasons:

1. If your delta v is 0, you're not going anywhere. Sure this is a degenerate case, but if it were truly irrelevant it wouldn't matter. More practically, your delta v is going to need to be at least your star's escape velocity (barring exotic scenarios such as an extremely close approach by your target star.)
2. Stars are not at rest with respect to one another. So at a minimum, you will require at least the delta v of the relative speeds of your origin and destination systems.
3. Even in generational ships, time to destination matters. Realistically speaking, half your delta v is basically your cruising speed (ok, this doesn't hold as you get up into the really relativistic speeds, but in that case we're probably not talking generational ships.) So delta v will directly relate to how long your journey will take.