I don't think I will be using LDAP for authentication. And I am not too sure exactly what each package does.
zimbra-core
zimbra-ldap
zimbra-logger
zimbra-mta
zimbra-snmp
zimbra-store
zimbra-apache
zimbra-spell
zimbra-memcached
zimbra-proxy
I just install whatever the default is.
You do want ldap, it is used internally for a bunch of stuff.
You still need LDAP for internal schema management, even if you're going to connect zimbra to an external directory provider. You can probably skip spell and proxy, but they aren't what generates serious loads, so might as well go with the defaults.
I'm running Zimbra 8.0.6 community edition and I had similar questions when I first did the installation.
After talking with a bunch of folks on the Zimbra forums and installing several instances of Zimbra myself, really you're going to want to install all of the default packages. However, zimbra-proxy is one that you can safely forget about unless you really need it. I guess you could also get rid of zimbra-spell, but I'd personally rather have it in there.
Hopefully your Zimbra installation went without a hitch. It's a great piece of software and a good alternative to Exchange.