I would think past tense, as a case study is usually about something that has already happened and you are interpreting/analyzing the results and making suggestions in the conclusion.

But here is an interesting view:

All the Case Studies and examples are described in the past tense, including programs that are still operating. 

If the program is still operating, the Case Study SUMMARY is written in the present tense.

http://www.toolsofchange.com/en/case-studies/