[Exim] Smtp-auth for localhost?

Hi!

If i am using exim 3/4 or any other MTA, is smtp-auth necessary for localhost
also?
Or do people keep smtp-auth for localhost itself too.

If i have relaying disabled does SMTP-auth helps in any way for the local machine.

Are there any performance or speed issues if SMTP-auth is enabled i.e. all the
clients _should_ authenticate thru various allowed smtp-auth mechanisms.

Please reply to this basic question because i will not find the answer to this
in any of the docs.

Thanks!

Regards,
Anand.
p.s.
Enabling SMTP-auth for everything will mean sending mails thru php mailer on
the localhost will also need password??Right? Is this really required in any
scenario. Or logical?