In general, anyone who is allowed by the system to create a tag may do so freely and without asking permission (either on meta, in chat, etc.).
The problem, as you note, is that other users might object to the creation of the tag. Such users can just as easily remove the new tag, possibly resulting in an edit war. In that case, both the tag creator and the objector(s) should search for and link to a relevant meta discussion (either a discussion regarding the specific tag or a class of tags in which the disputed tag falls). If a relevant meta discussion exists and the community's consensus is clear, the onus is on the party which disagrees with that meta discussion (whether the creator or objector(s)).
In some cases the community's consensus is not clear (either there is no relevant meta discussion, or there is an existing one but there is no clear consensus). In that case, the onus is on the party which is outnumbered in the initial dispute. For example, if the tag creator and one other user agree that the tag should exist but one user objects, the onus is on the objector. If there are two objectors versus one tag creator, the onus is on the creator. If the two parties are equal in number, the onus is on both to initiate the meta discussion.