My understanding of this torquing/ axle business is a GD&T thing:
There's a bolt hole pattern on the swingarm and a bolt pattern on the pumpkin (maybe I have that reversed). In order to assemble, the holes need to be at least as big as the bolt diameter, and with the exact same spacing as the bolts. Nothing in life is perfect, so there's some slop (tolerance) designed in: the holes are a little bit bigger than the bolts. This allows the parts to easily assemble and allows the spacing pattern of the bolts and the holes to be a little off-perfect and still go together.
But, this intentional slop means that the parts can be installed with different orientations to each other. IOW, bolt the pumpkin VERY loosely on the swing arm. Grab the pumpkin and twist it clockwise until it stops. Now twist it the other direction - it will rotate a tiny bit until it stops against the bolts - the amount it rotates is a function of the slop between the bolt and hole sizes and locations. This little bit of angular difference projected out to the other end of the axle is the issue.
You could also do this sliding up-down instead of rotating, and project an offset thru the axle; or sliding left-right effectively making the axle shorter or longer.
So getting the axle installed first means the wheel will force the pumpkin into the position that has the least stress. In theory, I'd guess this would be best accomplished with the rear wheel being made perfectly perpendicular to flat ground without any weight on it. In practice, this is difficult to achieve but my guess is that on the center stand is the best compromise between nothing or on the ground.
Keep in mind, this is really nit-picky stuff. I'm sure thousands of pumpkins have been removed and re-installed without any consideration of this and have been fine. The magnitude of errors and stress we're talking about is most probably negligible. I think it's one of those best-practice things that over-thinking pedants like me just feel better about doing.
I try to "worry less, ride more," but I have a harder time with "think less."