If you plan to be a DBA for a significant portion of time, invest the time in learning about query tuning.  I would recommend you spare the week to take a class if you can get your company to pay for it.

It will do two things that are really important.  First, it will teach you how to performance tune and troubleshoot slow queries in your databases.  Second, it will give you a much better understanding of what SQL Server is doing with the query you have passed it so you can more intelligently make decisions on what to do and what not to do.

You may never specifically use performance tuning and query optimization, but understanding how SQL creates execution plans will be a great help in database design if you have to build anything new.