A. Remember that the RID master is responsible for allocating RIDs (in batches of 500) to all domain controllers (DCs) in a domain. If the RID master is incorrectly restored (e.g., from an old backup), it might assign RID pools that it has already issued, resulting in duplicate SIDs being created in the domain. Therefore, I recommend that you give the RID master Flexible Single-Master Operation (FSMO) role to a different DC instead of restoring the RID master.
If you do restore the RID master, be aware that if you have more than one DC in the domain, the RID master must be able to contact one of them before its RID role will be started. In a disaster recovery situation, this requirement might be a problem because no other DCs would be available. Microsoft documents the steps to work around this problem at http://support.microsoft.com/?kbid=839879.