Lean towards relating two tiddlers with a tag if one is {part of} the other, and towards relating them with a link if one is merely {related to} the other.

Grok TiddlyWiki

I usually think of tags in relation to a blog where they indicate a relationship as opposed to membership. This is helping me think about organisation of information. I use a few TiddlyWikis, but have just stumbled through the process.