I ride my bike all the time in South Central Los Angeles, Compton and all over the worst neighborhoods. I know most people are scared of the crime but I would say the biggest threat to a cyclist is the drivers. They are the WORST more RECKLESS and DANGEROUS drivers I have ever seen.
As far as crime....being on a bike is much better than being on foot. Just keep moving, don't stop anywhere. If you see a group of males or males acting suspicious give them a wide berth.