I am of the opinion that we should create a designated list of tags that can be used. And of course, at the same time, leave the possibility of further enhancing our tag inventory to discussion here at the meta.
When entering or editing tags, the system will help by auto-completing suggestions based on what you've typed and the selection of tags in the system.
Creating a new tag is a privilege that is given based on earning reputation: https://genealogy.stackexchange.com/privileges/create-tags
During the private beta, this privilege (and several others) are set to 1 reputation so that all users can use them to help facilitate building the community and the initial content of the site.
During the public beta of a site (which generally happens after a week or two), the reputation needed for various privileges will be raised. This also happens farther down the road when sites graduate.
Requiring rep to create new tags encourages the use of existing tags. As the site continues to grow its content, the need for a new tag will not be as necessary. And of course, when a new tag is necessary, users that have earned the privilege and moderators can edit questions to include the appropriate tag.