Indeed, you either tag from the get-go expecting to have many, many files in the future, or you give up on ever contextually managing those files outside of large containers.
How do you tag a file when you can't find it to tag it, and when do you tag a file that you've forgotten about?
Ultimately, someone comes up with yet another abstraction that makes it just a little bit easier... Now, if we made every application those wrote a file also tag it meaningfully, and then had meaningful translations, and.. oh geez. I normally just delete everything and start over when I realize I have no idea what 90% of the files I just scanned were for. If they were truly important, I would've known what they were. I guess the people with ten bazillion files on a PC are just data hoarders. "But, but, but I'm going to need that report one day!" (Bet you would tag it, now wouldn't you?)
When you reach many, many files, just try to get people to tag them. Just try.