Have you tried utilizing the Data Advisor feature? I allows you to pick reporting attributes and provides details on what out of the box reports are available. Instructor Debbie taught me this one. I can't take credit. :-)
Depending on your version of Apptio it will appear in the main login or under the gear dropdown next to your name at the top right of your screen. You will want to navigate to Apptio Value Explorer and make sure you have chosen "What Apptio Does and & Answers" under the pickers. (as seen in the screenshot) I hope this helps with answering how to find the best reports to look at.