The tag mysql-5 naturally refers to mysql. Since the current production release is MySQL 5.5 (5.5.8 being the first GA), tagging questions with mysql-5 or mysql is acceptable.
However, keep in mind that there are still thousands of users in the world using older version of MySQL.
Such versions include GA release of MySQL 5.1
- 5.1.30 being the first GA
- Some are using 5.1 releases prior to 5.1.30 in production at their own risk
- Some are using MySQL 5.0, 4.1 and 4.0 and even 3.23.58