It won't automatically add tags unless you include it in the "list" to be included.
For example, I use the "NFL" as a tag to automatically be added in a particular forum. But, I also have the "Cowboys" as a 'auto tag' to be used if/when the word "Cowboys" appears in the title or content.
The tags will not auto-tag unless you have the text listed as a tag that you'd like to use. Hope that answers your question.