Updating to Drupal 7.x-dev until next stable release
No you are not: the Drupal core development workflow makes minor versions just tags of the main 7.x development branch. Periodically, the branch maintainer decides enough things have been committed to the branch to make a release worthwhile, and a new minor version release is packaged.

So let's say you download a snapshot of 7.x-dev dated 2011-08-15. At the end of the month, Drupal 7.8 is released. Drupal 7.8 would contain all of the changes you had received in the 2011-08-15 snapshot, and anything else committed between that point and the end of the month. It's essentially the same as if you downloaded a new development snapshot on 2011-08-30.

So, if you need to use the dev branch, feel free to do so with relative impunity.