NAS/Plex box, OS on RAID, or SSD?

The Project is coming together slowly, bit by bit. Parts are on order, build will commence next week. Storage will consist of 3x3TB WD Red drives in RAID5. I can cannibalize another system that's entirely good but sitting unused at the moment for an SSD. I'm undecided about whether I should, however. OS will be Ubuntu Server LTS, file system will not be ZFS. Is there much point to putting the OS on an SSD in a use case like this?

Considering youre ideally basically running steady state 24/7, not much sense in putting an SSD in there. That said, it wont exactly "hurt" either.