Quests that REQUIRE Flying

[Ayuhra]

You will pretty much require flying at level cap to get anywhere in a timely fashion and a lot of quests do ultimately require you to have it (a good deal of Sea of Clouds for example).

You won't be able to traverse most of Sea of Clouds or Azys Lla for any reason without flying.

I'd say just go for unlocking it, then you'll have it. I personally enjoyed the unlocking process but even if you don't like it, it's really not that bad.

Once you have it you can go back to those areas for Relic grinding on FATEs, gathering, rare mobs or whatever else you might find out you need later.