The Navy has no such guarantee in writing, but often (not always) accommodated in practice.
Yeah in my (I think *our*) time, I never saw #1 not get what they wanted. Probably even the top 5 got what they wanted. It's been a long time, but 72 NSS seems to ring a bell for what got me there, and there were people with less who did too. It's probably in a post of mine from 15-16 years ago if any nerds want to go looking. But to be fair, I wouldn't say that making it through the T-45 advanced syllabus was a high point in my life......I still remember my check in meeting with the VMFAT-101 Training Officer, who did not mince any words when he looked at my jacket and said "statistically, you will not make it through our FRS" I think by this point in my life I can say that his comment or advice was pretty misguided and wrong, but that is to say that primary NSS means basically nothing in life, and probably doesn't even mean much in primary.