SQL SPN

I had a few general questions about SQL & SPNs:

1. Do I have to register the SPN of SQL?
2. Does it matter if the Service is running under LocalSystem or a Domain user?
3. Despite the fact that I registered my SPNs I nevertheless see in the SQL log and error stating:The SQL Server Network Interface library could not register the Service Principal Name (SPN) [ MSSQLSvc/SERVERNAME:PORTNUMBER ] for the SQL Server service. Windows return code: 0x2098<c/> state: 20. Failure to register a SPN might cause integrated authentication to use NTLM instead of Kerberos. This is an informational message. Further action is only required if Kerberos authentication is required by authentication policies and if the SPN has not been manually registered.           Is this OK? Is there a reason?