When you add xcode4.5 to a question, that is the tag that will be added, unless xcode4.5 is not set as synonym of another tag, which is not the case.
Users looking for questions tagged xcode will not find a question tagged xcode4.5, if the question is not using both the tags. That is why it is generally suggested to use a version tag, and the generic tag. It is like with Drupal questions, which are tagged with drupal and a version tag like drupal-6.
Users can search for questions using a tag starting with xcode, thought, by using [xcode*] as search term; the search will be expanded to include all those tags, including xcode-project.