I used to like to make them very important until I saw a sort of moral decay set in and watched as players began to treat the NPCs like garbage and do meta things to protect each other even when they shouldn't have. This sort of PC's Only attitude leads to a cardboard world in which the players know that their character (or the next one if the rules kill them) is all that matters. I think it is important to have a living, breathing world beyond the PCs that goes on despite what they are doing, but that they can affect if they apply their characters to events in a way commensurate with whatever is going on. Because of this I always apply the test of whether or not the NPCs would consider the characters as important as they consider themselves, and if they actually have influence or they just want to have influence for convenience or whatever. To me though, it's dangerous for any game that will last longer than a few sessions to let the characters be Main Characters in a Movie. They tend to act more like spoiled actors than main characters at that point. How dare that NPC defy me!!

Before my game ended, my party was to be galactic saviors. They were finally getting to the good part with enough assets to be able to galaxy hop and were 2 - 3 sessions away from starting their stronghold acquisition mission.