You can end subsystems ahead of time if you want to, but it is not necessary.
PWRDWNSYS *IMMED will termineate everything and shut it down.
Just be sure nothing is running that you do not want to cancel.
Do <i>not</i> use the *IMMED option. Per my IBM CE when *IMMED is used the system does not write the cache contents to disk. Use *CNTRLD with a delay of a minute or so.