Start by believing; start by disbelieving. Both sound wrong-headed to me. Surely you begin by investigation, with  judgement suspended while you gather as much information as you can. In the Ghomeshi case, the prosecution "started by believing", to the extent that they failed to adequately investigate their own case and prepare themselves and the complainants for cross-examination. The result, an acquittal for Ghomeshi, whom I still believe to have been guilty as hell.