Hmm... post #478098 is the only such case at the moment.
My understanding was that media tags were for the genuine thing, and only when you're really sure -- following the convention for the traditional_media tag.
Semantically, it doesn't seem strictly correct to use, say, watercolor for both real watercolor pieces and faux "watercolor-ish" effects. Is there precedent for a tag being used with a looser definition than what it literally means?
In practice, though, as Xabid points out, searches can be refined accordingly. Is the use case of searching by specific faux medium common and/or important enough to warrant using the loose definition of the tag? Considering that the alternative would be to have a whole host of tags like faux_watercolor, faux_graphite, etc... which is just overkill.
A potential issue, though, is incomplete tagging: if you came across an image with watercolor but neither traditional_media nor faux_traditional_media, what to make of it?
All that semantic stuff aside, I'm also a bit uncomfortable with the notion of tagging faux_traditional_media images for trying to look like a particular medium. Some cases may be obvious, but in general, without some form of artist's commentary, can we be really sure? Especially with a painted look -- is it intended to be watercolor, gouache, acrylic...? (Heck, if not for medium tagging on pixiv, we'd be having similar difficulties with genuine traditional media too.)