Fundamental problem: my models do not always have a specific axis pointing up. Thus when they are uploaded to Sketchfab, they do not get rendered exactly right side up.
Trying to look at my models after uploading leads me to discover these related/knock-on issues:
It is very frustrating to me that the viewer doesn't let me rotate full 360 every way. Specifically it prevents me from rotating the model upside down.
This would be less frustrating if there were any editing tools that let me do basic minimal mirroring/flipping through 90 and 180 degrees of models once they have been uploaded.
So either feature would be a great addition.