Docs on Custom Fields/Tags?

Proper use of custom fields? I don’t think there’s such a thing; hence the name “custom” fields. You can use them for whatever you want. However, here’s a page which gives an example of what you can use them for:

http://codex.wordpress.org/Using_Custom_Fields

And a list of plugins which expands upon custom fields can be found here:

http://wiki.wordpress.org/Plugin#metadata

As for custom tags…what do you expect your tags to do? Most tags which might be labeled custom simply reference functions from plugins.