A study posted here, Ya-Ting Chang, Ke-Chun Teng, Yu-Cheng Tso, & Shiuh-Jeng Wang, Jailbroken iPhone Forensics for the Investigations and Controversy to Digital Evidence, Department of Information Management, Central Police University (2015) analyzes whether or not jailbreaking an iPhone alters digital evidence. The study was conducted comparing iTunes logical extraction and the MSAB XRY extraction tool, testing both methods before and after jailbreaking.
Jailbreaking generally makes the extraction of digital evidence easier. The authors conclude that the jailbreaking procedure will not alter digital evidence on an iPhone. iTunes still generates the same number of backup files before and after an iPhone is jailbroken. The jailbreaking process allows wifi and mobile phone data to be accessed which cannot be extracted if an iPhone is not jailbroken. iPhones automatically synchronize and download the most recent 50 emails from servers to which they synch. The emails can only be recovered if an iPhone is jailbroken.