I use the 'Old' tileset, which I believe best represents the Classic style. In my view, Classic should be considered as only actually having one tileset; the other two are simply skins for those with a different preference.
We shouldn't be making guarantees about whether everything will display perfectly on those tilesets, though I guess things could be improved by ensuring the tiles in each of them are at least visually equivalent (ie: a tile which is a castle corner piece in one tileset should be a castle corner piece in all 3 of them).
