From my experience (and reading up on the topic online) defragging a RAID is not an issue, though the defragging can go slow if caching options are not setup properly for the RAID. In either case it will not destroy data.
I believe what can be an issue is doing a ScanDisk (drive repair) on any striped RAID which would be RAID 0 or RAID 5 (and others but those are the common ones). So a RAID 1 (mirror) is OK even for ScanDisk.
Someone please correct me if I'm wrong, I don't want to mislead Ken!