RAID (mdadm) - Flags necessary?

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http://askubuntu.com – As I understand it, it's best practice to set the drive flags as "raid" before adding them to a mdadm raid array. Okay fine, but I also heard people say that they didn't do this and mdadm worked fine. Does mdadm auto-add the raid flag once you add the drives to the array and begin building? Is there any benefit to using the raid flag since it appears mdadm seems to work with or without it? (HowTos)