Technically "half naked" is meaningless in this game. A character is either naked or they are not. I do not tag for half naked.
The loss of the old forum wiped away a series of posts I did on choices of what pics to keep or delete, but one thing I covered there is that, if I have a series of pics clothed, topless, bottomless, and fully nude, I will generally toss the half naked ones, because they serve no purpose on their own, and all useful tag combinations are covered by the clothed and nude pics. A half naked pic (either way) and a fully nude pic would get the same set of (non-extended) tags, and the fully nude pic is IMHO better then the half nude pics. Maybe if I have a girl with only very limited choices of R34 I would use them, but usually I have enough that that is not needed. I will add that I only toss them when I have a fully nude option from the same set. If a topless pic is the only one I have like it, then I definitely keep it.
As for the "panties" tag, I often use it for panty-shot pics. And yeah, if it is the only thing she is wearing I may tag it, but that would be because of WYSIWYG, not because I'm trying to signal half nakedness. But being a extended tag it's a lower priority tag, and I often enough just forget to even set it.