Should you use RAID on a backup server?
For redundancy, I would not
If you are not in the habit of restoring specific revisions of files of your backup system and you fear that you might have to do that in case the backup system disks fail. Then yes, I would use RAID 5 or mirroring or even striping and mirroring.
The only reason to do this if you expect that the original data might be unavailable at the worst possible time.
For expanding disks into one volume (striping)
Maybe, but be aware, that if one disk dies the entire array dies.
I think it is a better practice to backup the backup server. I know it sounds silly, but bear with me. Backup the system disk, the configuration files and the backup settings. That way, if your backup server fails you can be up as soon as possible.
(Edit: sorry, about the other answer, I misread the question)