Yes, this is necessary if you want a graceful shutdown of your servers (and to minimize risk of lost play).
MineOS already offers the auto-save interval in the webui, but obviously even the smallest amount of unsaved work is undesirable. As demonstrated in the link @iMelsom provided above, you’ll want to create a script that will run on the cronjob rather than add commands directly to the cronjob.
Also see this link for ideas on what you would execute to stop the servers:
cd /usr/games/minecraft
./mineos_console.js -s myserver stop
The reason you don’t want this in a crontab directly is because (while possible) it is less clean and straightforward to have each of your commands execute in the order you want it to. If you have as script, e.g., “shutdown.sh” that changes directory to the Minecraft scripts directory, shuts down the servers, (waits for servers to definitely be down), and then tell the OS to shut down, it’s much more reliable than simply having each happen, say, one minute after eachother in crontabs.