Is it necessary to stop SQL services before reboot the server?

>Is it really needed to STOP SQL Service before reboot the server?
no.

If the SQL service does not stop before the timeout value, the operating system will kill the process. What happens with the transactions running at that moment?

all transactions running at that moment will be rolled back.
you should indeed ensure that there are no critical transactions ongoing, resp tell all end-users about the reboot.

In an ideal world, you would take the time to do an orderly shutdown os SQL Server - it does a checkpoint at that time.

In the real world, then the reboot is often performed before shutting down. 99% of the time, it won't matter. If it is a planned shutdown, then chances are the users aren't logged on (or shouldn't be). The reboot process does shut down services anyway...

I'll give you an example of a scenario:

I shut my server down.
The server shows that, during the shutdown process, SQL Server service stops responding.
So, in the background, the Operating System starts counting down, based on the value in HKLM\SYSTEM\CurrentControlSet\Control\WaitToKillServiceTimeout.
If SQL Server service does not stop timely, I assume that Windows will kill sqlserver.exe. And that could be risky.
If I stop the SQL Server service manually, WaitToKillServiceTimeout won't be used, because it's only for the shutdown process. It means that, if the service takes more time to stop (bigger than the value WaitToKillServiceTimeout), it will stop gracefully.
It's only a thought... What do you think, folks?