A general unwritten rule (or what I believe is one, since I haven't been able to find it mentioned anywhere) is to tag characters if they appear as dolls, small keychains, chibi, an icon representing them, anything as long as it's recognizable. For example post #5438900, post #3409251, post #4332618, the entirety of character doll, etc.
(This has also been partly discussed in topic #15982, because an artist kept adding many small characters to his artworks, and tagging them polluted searches.)
However, although the aforementioned rule also applies to cosplays, it is only applied sporadically for when a character's clothes appear.
post #3891684, post #1491381, and post #828442 show recognizable clothes of other characters and yet aren't tagged, while post #4410756, post #4579131, or post #5023566 do.
Another odd example that comes to mind is characters clearly interacting inside the artwork, but being out of frame, such as post #4735661.
It would be nice to have clear guidelines on the approach of tagging characters without the character necessarily appearing.