[throwawayiionqz]

Why tag anything at all? If the query is fast you may run the query again. This avoids migrating tags if the query gets modified/improved etc.

[em-bee]

i use tags for annotations and actions: this email i need to reply to. that one links to a story i want to read later. etc. it's not possible to search for that without tags, because those states change and don't depend on the actual emails

[nextos]

AFAIK, you need tags at least for read/unread.

I don't really use tags, but have a few searches to archive email from mailing lists without seeing it in my inbox.

For other usecases, I do as you suggested.