If you have the ability to travel between stars, it doesn't seem like there's still need to conquer a planet for resources - except for resources that aren't natural.
For example, you can easily just get water from comets/asteroids, mineral resources from uninhabited planets (easier than to fight the inhabitants), and gas/fuel from the gas giants.
However, one resource you cannot get is slaves. Therefore, i predict that any advanced civilization that can travel between stars would actually invade to capture the life forms as slaves, rather than to extract resources.