Unity doesn't use the local administrator account. We do place the domain level Unity service accounts in to the local administrators groups to allow access to various resources on the Unity server.
None of the Unity accounts should have domain admin permissions. People often add our accounts to these groups and it is 100% not necessary and in a lot of cases can actually negatively impact our effective rights.
This URL has the specific required permissions for the accounts:
http://www.ciscounitytools.com/HelpFiles/PW403+/PWHelpPermissionsSet_ENU.htm
Any deviation from these permissions isn't supported by Cisco. This includes placing the accounts in any groups.
Let me know if this answers you question or not.
Thanks,
Keith