I started thinking about it because the premise that this life is a continuation of the pre-mortal life and that the state we are born into is a result of our valiance and effort in the last life.
I believe in a pre-mortal existence, but I think the idea that the state into which we are born into mortality is a result of valiance and effort in that existence stems from a faulty foundation and is a mutation that stemmed from the earlier attempts to justify the Priesthood ban and on-going attempts to claim that we in the LDS Church are special. I see both of those justification motivations as the result of pride, even as I understand how deeply attractive they are to the natural (wo)man.
I believe more in the concept of, "There but for the grace of God go I," than in the idea of, "There but for my superior diligence and righteousness go I."
Those are two very different premises, and they address belief in a pre-mortal existence and rewards for varying levels of righteousness. The first one (a continuation of life from a previous existence) in no way depends on the second one (some kind of valiance determinant), and the second one is only stated directly in our scriptures about "the noble and great ones" (in Abraham). It can be read as implied about others in places like Jeremiah 1, Alma 13 and D&C 138, but it is not an automatic and obvious implication unless that belief is held prior to reading those passages.
There is no indication whatsoever anywhere in our scriptures to support the idea that someone born into poverty or with a disability of some kind was less valiant in the pre-existence than someone born into wealth or full health - or, just as importantly, vice-versa. In fact, in the one case where disability is mentioned in the same passage as a pre-mortal life, Jesus said the blind man was born that way to manifest the power and glory of God. (and that passage is crystal clear in its portrayal of Jesus' followers believing in a pre-mortal existence and Jesus not contradicting that belief)
So, even if we take the scriptural accounts literally, which I generally don't do, the idea of varying degrees of valiance in a pre-mortal life affecting birth into mortality for the vast majority of people who have lived and now live just isn't there. I believe that we, as a people, needed a justification to deny black people access to the temple (not just a Priesthood ban), so our former leaders bought into the whole curse of Cain nonsense that was being preached in the Protestant congregations of their upbringings (since they all were converts originally) and used that to develop a uniquely Mormon version that expanded the apostate belief to include valiance in the pre-mortal life. We also wanted a reason to claim special status as individuals (as pretty much all religionists have done since the beginning of time), so we took the Protestant idea of pre-destination and the general idea of the noble and great ones being fore-ordained and morphed that into the idea that every person who is born or baptized into the LDS Church was fore-ordained because of pre-mortal valiance.
In short, I believe in a pre-mortal life, but I don't believe in varying degrees of valiance in that life that are exhibited in this life. I don't believe there were fence-sitters in the War in Heaven. I think our temple theology annihilates the idea of varying valiance (or, at the very least, that, if it did exist, it matters in any way whatsoever after this life), and I also like Bruce R. McConkie's statement after the Priesthood ban was lifted that said we need to forget every justification that was uttered by anyone, no matter who they were, to explain the ban - that "we" spoke from "limited light and knowledge". For me, that includes the idea that pre-mortal valiance played / plays any part in the objectively quantifiable circumstances of our mortal birth.