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.
For interstellar journeys at sub light speeds the size of the DV is largely irrelevant in terms of the time it takes to complete the journey.
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Using your example and assuming an upper velocity (which is not mentioned in your question btw) of .10C the journey is going to take approx **100 years** with the acceleration & deceleration phases added on.

From the perspective of the crew it doesn't really matter if the ship takes 3 years to reach its 'cruising' velocity or just 3 weeks. With latter you end up with a 100 year (plus 6 weeks) long journey. With the former it takes 106 years. For all intents an purposes putting the 'pedal to the metal' only shaves a measly approx 6% off the total travel time. Hardly a huge saving for a generation ship. And of course the time saving only worsens with increases the distance. Double the distance the ship has to travel to 20 LY and at the high DV your only saving approx 3% on travel time over the lower DV. And on it goes.

Plus a lower DV is an easier engineering challenge which you want wherever possible. *Everything* on a generation ship has to be engineered to last because if breaks down and cant be repaired with what you have on board you screwed.