Arguably the parity drives are the LEAST valuable, as when they fail you have no data lost, and the chances of losing data then rests on the data drives.
Here is how to best visualize this. 2 parity drives allows for 2 simultaneous drive failures, the 3rd failure will cause the total data loss of all failed drives. So, no data loss until the 3rd failure. If the 2 parity drives fail, and then a data drive dies, you ONLY lose that single data drive's content. If 1 parity and 2 data drives die, you lose both data drives worth. If, and here is the takeaway, both your parity drives are fine, but you lose 3 data drives, ALL the data on ALL three drives is gone, and the 2 parity drives can do nothing to recover any of that data.
So, best case scenario, in a 3 drive failure, is for BOTH of the parity drives to die.
If you have less faith in a particular drive because it's not "new", then the best place for it is in a parity slot where it can do the least harm if it fails.
Now, the parity drives do get writes for every bit of data sent to any drive in the parity array, so I'm not saying use junk drives for parity, just statistically speaking you want the parity drive(s) to fail, not the data drives.