When is it ok to spin down a hard drive ?

Dear Lazyweb,

My home NAS has it operating system on a usb stick and data on four sata hard disks that I would like to spin down (so they are less noisy and save some energy). But what if I run an rsync on them every 20 minutes ? Or every hour ? Is it better to let the drives running because the spin down spin up cycle will cause excessive wear on the drive ?

When is it ok to let the drives go to sleep ?

I use this /etc/hdparm.conf for spindown after one minute:

root@gwen:/var/log# cat /etc/hdparm.conf
quiet 
/dev/sdb {
        spindown_time = 12
}
/dev/sdc {
        spindown_time = 12
}
/dev/sdd {
        spindown_time = 12
}
/dev/sde {
        spindown_time = 12
}