Additionally, Takahashi Rie actually describes at least two things: posts that have Takahashi Rie, the real life CV, and voice actor connection posts that don't depict the person at all. So there should also be a takahashi rie (topic) general tag for most of these posts, else you can't actually search for what you want. post #7270696 is basically the only takahashi rie tag that actually has the real life CV.
touyama nao is a better example. touyama_nao chartags:1 reveals 7 solo posts that depict the actual CV. There are a few other real life posts where she is also depicted with other CVs. Some are tagged wrong. There are also posts where she and her CV idol group are cosplaying as their characters. Or the characters appear along with their CVs. There are also solo and group posts where the character she voices is dressed in her idol group's uniform like post #6287225 (does this count as touyama nao (cosplay) too? maybe; I didn't think to tag it). There are posts that are only tagged Touyama Nao because they're celebrating the CV's birthday, but it's just a drawing of her character.
This all makes it very hard to find posts that are actually of Touyama Nao. It may be best to keep it simple and separate it so you can find what you're searching for. It's easier to tag that way. I think this makes a lot of sense for the CV char tags and (proposed) CV gen tag.
For tagging literally every instance of takahashi rie's voice with Takahashi Rie (voice) it would greatly simplify tagging and searching, even if it feels excessive. Ultimately, it is not really that big of a deal, because the vast majority of posts do not have sound. There are tens of thousands of posts depicting characters that Takahashi Rie voices but only dozens where her voice is actually present. So we could just tag them all.
When you consider ALL posts that feature an industry CV's voice, it seems like it's much more of a tag bloat. But there are probably only thousands of posts that would "need" an audio tag added (it's fine if it's missed, really, just a nice courtesy), and that's just because there's a backlog of posts that haven't gotten the treatment. It is not that annoying to see two artist tags on a post with audible dialogue, and it would have a practical purpose. The post has audio, so you might be curious of hearing more audio of the character. Or you just want to know whose voice it is. Saves you the step of looking it up, and CV isn't always mentioned in the post/commentary (it's often written in kanji and may have to be OCR'd, anyway).
I could go either way, really: having confusing guidelines like my 7 examples, or just keeping it simple and tagging what you see and what you hear, absolutely every time. The former is cleaner but confusing to tag; the latter is more useful to users and simple to tag.