I'll try to shed a bit of light on this, from a functional standpoint. Simply put, there is ZERO requirement to have a VA disability rating for a condition when seeking to add it/having it considered during the IDES/PEB process. In fact, unless the diagnosis in and of itself causes a Service member to be unfit (yes, there are a few diagnoses that do), the PEB won't bat an eye unless you can show the condition causes funtional loss. Nevertheless, if the unrated condition causes you to be unfit and the PEB determines it so, the PEB will request the VA rate the unfitting condition before it renders its final decision. To be clear, I have not commented on the evidentiary weight of VA ratings during an IPEB, FPEB hearing, or PFR (USN/USMC), as that would require much more discussion and analysis of case-specific facts that would be inappropriate on a public forum.
Therefore, if you stay silent and don't request an FPEB or PFR on the basis of not having a VA rating, you will likely not have standing to appeal to BCNR later, as you didn't exhaust your administrative remedies.
The above discussion notwithstanding, if the goal is to avoid post-IDES VA complications/have accurate ratings upon exiting the military, filing a new VA claim during the IDES makes complete sense, in theory. Of course, in reality, there is always risk that the initial ratings will be reviewed and changed. Although I cannot comment on the magnitude of this risk, the decision to shoulder this risk must be made with extreme care and close attention to the facts of your individual case.
I hope this helps.