I would say, it depends on you use case, and your pool redundancy.
If you are doing light work an the RAM you have now is working good, then don’t mess with it.
If you have a RAID-Z2 or better and the NAS is not located is a remote location where you cannot access it in a day to replace a drive, then you should not need a Spare drive.
I’d rather have a RAID-Z3 than a Z2 with a spare.
It comes down to what makes sense to you.