Are you running into some kind of problem? Have you tried yet, or are you just asking?
I’ve never used Syncthing, so I don’t know. But what I do believe is that syncing tools should not be relied on as a backup!
I suggest making sure you backup your vault first. And then just try. If something goes wrong, you can restore from the backup and ask for help.
Ryan put it well:
In a basic setup, you don’t need to close Obsidian. External edits will automatically get picked up. If you’re editing them at the same time on multiple devices, that can definitely cause problems.
But in practice, especially with cloud services (and it seems iCloud causes the most problems), things can go wrong. But usually don’t. Either way, I know this doesn’t directly answer your question: backup your vault!