I use tags on my friendly Discourse, because everyone there is encouraged to treat it like Wikipedia, be bold, and higher trust levels are actively encouraged (we are a small group, 50 users total) to edit posts for a better experience; sometimes that means tags. This same instance I use as a database to feed static sites content, so I use tags for that.
On my worky Discourse we don’t use tags. Projects get their own categories, and search does the rest.
On my personal Discourse I don’t use tags. They’d be just for me, and I have broad categories to serve as inboxes.
On most other Discourse instances, no tags. I don’t think they are useful unless they are intentional, meaning prepared tag groups or a community spirit that embraces them. Tag groups work really well for company support forums, where there are many sub-products, for instance.