I don't even think it's necessary to reserve deck chairs.
If you're with a group all sitting together, and someone needs to leave for a short period of time ... either to use the pool or the restroom ... your companions can readily tell someone "eyeing" that chair that it is occupied. No problem.
But if you're planning on leaving the chair for hours ... either to go back to your cabin to nap or whatever ... then it is simple courtesy to "release" that deck chair for someone else's use in the meantime.
Blue skies ...