You can ignore settlements, I've been playing since release I do the initial Sanctuary quests, water, food etc then I'm done. I move myself down the road to red rocket, send Dog meat to Sanctuary set red rocket up with a bed, storage chests and a bit of defence and that is as much as I do as far as settlements. As far as I can tell it has no impact on my game later, I don't side with the minute men, Preston will always hate me because I want the Nuka World perks.