Local System is pretty minimal, safe for a local SQL instance
If you need Network-features such as back up to UNC, talk to other servers etc... use a domain account if you can
You don't need Local Admin
This is for SQL 2005 Express, but applicable as wellhttp://msdn.microsoft.com/en-us/library/ms143170(SQL.90).aspx
SQLServerNewbieMCITP: Database Administrator SQL Server 2005
Use the built-in System account
You can assign Local System, Network Service, or Local Service to the logon for the configurable SQL Server services.
Local System account
The Local System option specifies a local system account that does not require a password to connect to SQL Server on the same computer. However, the local system account might restrict the SQL Server installation from interacting with other servers, depending on the privileges granted to the account.
Local System is a powerful account. It might not be appropriate for all service settings. For more information, see "Security Considerations for a SQL Server Installation." in SQL Server 2005 Books Online.
Network Service account
The Network Service account is a special, built-in account that is similar to an authenticated user account. The Network Service account has the same level of access to resources and objects as members of the Users group. Services that run as the Network Service account access network resources using the credentials of the computer account.
We recommend that you do not use the Network Service account for the SQL Server. Local User or Domain User accounts are more appropriate for these SQL Server services.
Local Service account
The Local Service account is a special, built-in account that is similar to an authenticated user account. The Local Service account has the same level of access to resources and objects as members of the Users group. This limited access helps safeguard the system if individual services or processes are compromised. Services that run as the Local Service account access network resources as a null session without credentials. For more information on service accounts, see Setting Up Windows Service Accounts in SQL Server 2005 Books Online.