[Verizon Galaxy Note 3 Q&A, Help & Troubleshooting] bftb0:
Neither of those alternatives.

The "AR" (Anti-Rollback) behaviors that are enforced by whatever bootloader version is on your phone (MI9/MJ7/ MJE/NC2/NC4/NK1/OB6/OF1) mostly affects whether you can flash prior versions of bootloader firmware, not "ROM" partitions.**

By ROM I mean:

boot,system,userdata, and cache partitions.

So look, Samsung would obviously prefer that both their bootloader & ROM software stayed in tight lockstep. And that would be the case if there were no rooters/hackers.

But you don't use Odin to install custom ROMs; mostly a customized recovery boot is used, so Odin doesn't get to interfere with that. So the technical issue boils down to these three points:

A) Will the bootloader on the phone successfully be able to successfully load & execute a boot.img (kernel+ramdisk) from a given version of ROM?

B) Will the mysterious TZ (TrustZone) processes associated with the bootloader firmware interoperate correctly with the kernel being booted, when the kernel & TZ might have origins in different releases?

C) Did Samsung plant any logic bombs in their kernels that try to enforce lock-stepping of bootloader release version with kernel release version?

The answers to these questions are not precisely known; empirically it appears that the answer to (C) is "No", and a partial answer to (A) is that the M* series firmware releases are not able to boot N* and O* kernels due to technical reasons, but there isn't (yet) much investigation into whether newer bootloader firmware (N* and O*) can boot older ROMs (backward compatibility). I asked this question the other day, but I guess nobody is interested.

To answer your question more directly though - if you were to flash a 6.0 ROM it will either boot or not depending on the version of the bootloader firmware you are using; and the same thing is true if it were a 5.0 ROM. But the ROMs don't alter the bootloader firmware, so as long as they are both bootable you should be able to go back and forth between them with the appropriate sequences of backup/wipe/restore ***

** The bootloader AR prevention is not strict as in version-by-version. e.g. I believe you can roll backwards and forwards between any of MI9/MJ7/MJE/NC2 (?? & possibly NC4 ??). It might be possible to go back and forth between OB6 & OF1 but I am not sure. There are specific points in the release sequence where AR fuses are blown, preventing rollbacks prior to those specific points... but not at every release.

*** there is no theoretical reason why this is true. There's crap on these stupid Samsung phones which is not reinstalled during ROM flashing or firmware flashing (e.g. efs, pers, persdata partitions and others) containing data that could conceivably go through an irreversible upgrade in format during a ROM upgrade - or get written in a "new" format by later versions of Stock ROMS while they are running, rendering them non-working in prior ROMs. Since Samsung tries to enforce Anti-Rollback, there's no reason these kinds of behaviors could not occur - Samsung isn't engineering things with an expectation of permanent backwards compatibility. So the evidence that this doesn't happen is largely anecdotal - nobody has uncovered buggy behavior that could be tied to such a mechanism.