From experience I learned that every hard drive will fail, it's just a matter of time.
I have learn my lesson the hard way and now I do backup.
When I bough new drive, I often segregate my drives list with the warranty period. Hard drive manufacturer are there to make money and obviously, most of the time, they designed their hard drive to last at least the warranty period. So after that period, I expect the failing rate to be greater. I already had 2 of 3 drive of a RAID 5 failed almost at the same time (second drive failed when reconstructing the array and yes I had a recent backup).
My question is: What is the best practice with preventive replacement of hard drive in a RAID after the warranty?
Do you care about it? How many drive in the array do you replace?
Notes on responses
When creating a new array: use drives from different manufacturer / batch.
When having an already old array: add a new spare.