Best use for RAID1 spare disk?

I have a PE 1950 with PERC and a pair of disks for RAID 1 (mirroring). I want to get a spare disk to keep on the shelf, so if one of my RAID disks die I can pop the spare in. Here's my question. Is it best just to leave this spare disk collecting dust on the shelf? Or, should I rotate the spare disk in on a monthly basis? In other words, I start out with A&B in the server, and C on the shelf. Next month, I pull A out and put it on the shelf, and insert C which the RAID will automatically rebuild. Next month, I pull B out and put it in the shelf, and put A back in. And the next month, back to A&B, with C on the shelf. Pro's for the rotation method: (1) There isn't a spare drive that just sits forever on the shelf - and then may be dead when it is needed. Instead, all 3 drives are used for two months out of every three, so a drive failure is noted and dealt with. (2) Wear & tear is evenly distributed for all three drives, so I hope that failure is postponed by a certain number of months. (3) The extra drive on the shelf is a complete "backup" of the raid as of less than a month ago, and could be used to boot the system in case the two internal drives should both fail at the same time. (Not impossible - a lot of external events that cause a drive to fail could cause both to fail, like a surge or a fire.) (Doesn't replace daily tape backup, but is a nice addition.) Con's for the rotation method: (1) Causes a full rebuild once a month, causing extra wear & tear on disks. (That's all I can think of). Any ideas or suggestions? -Steve