Changing the disks on a Power Edge R510 -- Dos and don'ts ?

Hello,

We have a few Power Edge R510 ( 12 disk options, RAID-6 ). The disks are getting old and a few died this winter (3 total on a specific server).

I have little experience with maintaining systems with RAID arrays. So, I am wondering what is the best strategy to take:

1) keep replacing diligently the disks as they die 

2) replace all disks in one shot (we do have enough spare distributed space to do it)

I guess my real question is : at some point, are the disks going to die all together and put us at risk of loosing data even if I keep changing the disks as soon as they fail ? Am I gonna wake up one morning with 3 disks dead ?

Thanks in advance for your comments

Gilbert