Only time you need to mess with /etc/mdadm/mdadm.conf is with LiveCD when you need to access your underlying RAID and there is problem with it, so LiveCD is unable to mount it. There is explanation of this process somewhere on the internet.
Take a look at this examples , particulary 6. 7. and 8.
From Linux RAID Wiki
, section 1.4, RAID 5:
If one of the disks fail, all data are still intact, thanks to
the parity information. If spare disks are available,
reconstruction will begin immediately after the device failure.
If two disks fail simultaneously, all data are lost. RAID-5 can
survive one disk failure, but not two or more.
BTW, there are experts that recommend avoiding RAID 5 since random writes are very slow, and that two disk failures will bring the RAID down for good:
You might be better of with RAID 6. It has all the benefits od RAID 5, but it also has double parity and instead of having spare disk and waiting for RAID rebuild, you are actually using that disk to provide redunancy even if two disks fail together.