I manage a blog with 3 editors and 8-10 contributors.
In my experience, the best way to manage tags is to simply instruct contributors not to add tags to their own articles, and instead have the editors be responsible for tagging articles. You can then work with editors to agree on what the purpose of tags are on for your site and make sure they enforce those policies.
I use the Simple Tags plugin for adding tags to articles which replaces the default tag entry system. When you type in a tag, it automatically searches existing tags and displays them in an AJAX-y dropdown display, which is very helpful to avoid dupes and to see what similar tags exist.
I hadn’t always managed tags so diligently, so I used the Terms Management Tool plugin to merge similar/duplicate tags together. We had a lot of things like “Firefox” and “Mozilla Firefox”, so I combined those into a single tag, and Simple Tags helps avoid problems like that in the future.