If the back-end you're working on is SQL 2008 R2, then that version of Management Studio is just fine. It will also work on the SQL 2012 back-end for functionality that was included in SQL2008 R2.

The 2012 version of Management studio allows you to have different script tabs in separate windows, which can be useful if you have two monitors on your computer. I tend to click between tabs a lot when using the 2008 version. Just be careful which window is active when you click Execute.

If your goal is to pass Certification exams, I would recommend obtaining SQL Server 2014 and Management Tools however you are legally able to do that. As time goes on I would expect Microsoft to add 2014-specific questions to the exams.