localhost, 127.0.0.1, server IP, host name: which is best?

I realize this is a broad question if asked in the context of various daemons, services, scripts, etc. I'm relating to WHM's resolver setup, when it was adviseable to replace the local host IP (127.0.0.1) with the server's main IP for security reasons, which made sense. So when does it not make sense to replace localhost settings with something more specific?

Let's take my mail system as an example:

In squirrelmail/config/config.php, for the $domain, $imapServerAddress, and $smtpServerAddress entries, should 'localhost' be replaced by the server's host name or IP? When I look into it, some say to do it this way, others say to do it the other way, while nobody disagrees that 'localhost' will always work.

Another case in point are my courier-imap/imapd and pop3d files, which seem to be using 127.0.0.1. Would I be better off using the server's IP?

 

Thanks for the info.

If I use the server's host name in the imapd and Squirrelmail configs, will it speed up the lookup process? For now I've got DNS lookup disabled wherever it's an option. But mail in general seems too slow. Converting to maildir was supposed to help but I haven't noticed any enhancement over mbox in terms of speed.