I assume that eventually this case will be stood upright, not on its side as pictured? If it's going to be kept on its side, then I'd put a rubber shim right in the middle of the area below the socket.
If it's going to be stood back up, I'd put the shim at the bottom of the heatsink mount as that is where the leveraged weight will deflect the board inwards the most.
I wrote rubber but wood or stiff cardboard would work as well, with your choice of adhesive to the case plate... and of course what I mentioned above about mounting also depends on if it has a solid case plate or holes in it so you just have to get it as close as possible to the greatest inward deflection point on the motherboard.
I tend to disagree about heat and time. I suspect it will more likely be a problem when the system is cold, solder more brittle then, and damage could occur the moment you move the system and shift the weight, changing board flex and cracking a solder joint... assuming it isn't damaged already.
The other school of thought is just ignore it but I assume this is a new build and you're looking for best way to achieve long lifespan out of it?
I removed everything and screwed everything back on, including removing and putting back the backplate.
I/O panel is on now as well, and taking off and putting on the case standoffs.
Looks good enough to me now I guess.