I personally have never used the tag option. I was basing the use of the tag off the following examples:

 https://github.com/travis-ci/travis-ci.github.com/blob/master/docs/user/deployment/npm.md#24 

https://github.com/travis-ci/travis-ci.github.com/blob/master/docs/user/deployment/rubygems.md#L25

https://github.com/travis-ci/travis-ci.github.com/blob/master/docs/user/deployment/pypi.md#L26