I backup the entire photostructure folder, not just the .photostructure sub-folder.
Why? photostructure creates xmp (sidecar) files next to the picture for any metadata changed within photostructure. Right now the only metadata that photostructure edits/creates is image rotation, but that’s (hopefully) going to greatly expand in future releases.
@bdillahu thanks, that’s the post I was going to refer to as well.
I actually just edited the TL;DR: in that post to be a bit more nuanced:
Absolutely, positively back up your original assets. Whether that be backups of your original directories, or your new organized photostructure library, make sure one (or both!) have a backup.
If you have the space, you can back up the .photostructure directory, but if your library is large, the size of the .photostructure/previews directory can actually crash some backup software.