Actually 10 percent would be far too little in my view. If this RAID setup you have contains an operating system, then yes you need to keep lots of space free -- I would lean towards 20 percent, though "percent" is not really the right way to measure this need, it's more about how you use the computer and thus people tend to fall back on a comfortable percent because there's really no simple rule that works universally.
If you find yourself needing 12TB of storage, then I would presume you work on some big files, so your need for temp/cache/etc free space is probably way larger than a light-duty user, just as an example.
So anyway -- tl;dr version -- if you're booting off this, then yes you need to keep a substantive amount of free space available. If it's just data storage (no OS), then you can fill it right up.