I think in-depth analysis of this question would be a great thing! I think the progression that most of us take is that we start out repairing our airship fully even when it has about 60% durability left. Then as you get to the higher level parts you start to run out of materials, and since it takes the same amount of materials at 0% as it does at 80%, you start to be more conservative. You also notice that you are still getting high ratings sometimes and discovering sectors, which feels convincing enough to rule out repair condition for yourself.
Still, I will always prefer a systematic analysis to anecdotal first-hand experience. If you explore this question, let us know what you find.
Let me run an idea by you and see if it might hold some water. I wonder if the game determines the quantity of items first (using a base number, RNG, and retrieval as factors) and then assigns a rating based on that quantity (using cutoffs). Sometimes the difference between Invincible and Enterprise parts would be enough to push it past the cutoff and then sometimes it would merely just be higher but within the same cutoff. If that were the case, we would need to see a higher magnitude of difference between parts to really notice consistent differences in ratings. Hopefully as people get more creative with their endgame parts we might see this!
It is far from confirmed, at least as far as I am aware. It was originally asserted in the guide, but I also gave my rationale, which boils down to this: The three voyages after I upgraded aftcastle got A ratings, where I was mostly getting C before. The language in the voyage log seemed to support the idea that the retrieval stat boosts retrieval ratings, and retrieval ratings directly influence sector and voyage ratings. I wrote the guide just a couple weeks after the expansion, so we hadn't had time to gather much data. Thanks to Nyalia's spreadsheet, we have tons of data now \o/ We can really put these claims to the test.
Another issue, it now seems really likely that higher level sectors have more "difficulty" when it comes to retrieval, so even if you stay current with the latest parts, you will still see lower ratings as you visit more difficult sectors. So a correlation may not be obvious until that factor becomes more constant.
It is a really good point, I should definitely add something about this! (Edit: done!)