I had to read your post twice because I was about to disagree. Then I noticed you say 45*. That's quite severe, and very much impractical as this conversation goes. In such case, yes I concede you likely may be correct.
Otherwise, in practical real life terms, I have a bundle of cues ranging from old Titleists to some of my earliest cues standing up, leaning in the corner of my shop. I'm on my third shop now, currently TN, before in AK then NM. The cues have yet to warp. Going on 20yrs since my shop was in AK and the cues are still straight after spending that long leaning on the wall and traveling half way around the globe from frozen north to desert to wet south. Perhaps gravity will someday force a bend in them, but it hasn't yet. They are not leaning at 45*, but rather more like 2*-3*. In my personal experience, I have not seen gravity force wood to bow. Maybe others have. I have not.
That all said, take a wet board and lie it flat on your concrete driveway for a few days in the sun. Gravity will not keep it flat. It will lift itself off the concrete as it dries. IMO, gravity is significantly less force than the strength of a piece of wood to either move or stay straight.
Part of my thinking in attempting to solve a problem is to look at it in it's extremes.
Then, moving towards the center of those extremes is where the solution lies.
Yes, 45* is a bit severe but it was meant to be; that was one of the extremes.
The other is the perfectly vertical. Anything in between should be considered a variable.
Shafts by themselves should present no problem regardless of how they're stored.
Going on gut instinct (absent of documentation) what I believe comes into play is,
strength of the mtrl vs mass. A 13mm, 4oz shaft is pretty stout. If having passed all
critiques coming to that finished size, it doesn't matter if it's hung, laid flat or even canted.
It's already found it's 'happy place'. There's just not that much mass for gravity to play against.
Your example of the board on the driveway I believe is a different phenomena.
In this one, gravity loses. I see it as moisture in the cells of the top 1/2 of the board evaporating
while the cells in the surface in contact with the concrete are being sustained in their H2O content.
The sun doesn't see the bottom side so it's not drying-out. That's a real good way to induce a warp.
Just in case no one has picked-up on this, I will on occasion, deliberately make a statement that will
be questioned or 'called-out'. Why ? Because sometimes I don't know and am waiting for someone to tell me.
Another reason is that we are not alone. There are many readers of this forum who have never posted
and only read. For whatever reason, they will not ask 'that' question. By me putting myself on 'front street',
a bevy of responses will ensue and that mystery reader will now have numerous opinions on which to decide.
In that scenario, it's not just me that's learning, it's a lot of people that otherwise might not have.
Peace Out, KJ
PS - I just saw the upside-down milk crates. Very trick idea. I do the same thing.