I like to have at least 1 between the skimmer and pump - keeps you from getting soaked when dumping the pump basket. For years i just used a little foam football

and wedged it in the skimmer hole - it still allowed a bit of water through though.
It was glorious once i added a valve under the skimmer.
I also have valves on each of my returns. This allows me to not only isolate the pool from plumbing if needed but also control the flow to each return if i want when using my fountain or my pressure side cleaner.
I am about to hook up my plumbing for the season and all the valves mean that i can do so at my leisure & just turn them on when ready.