Sometimes it helps, riding through a high-crime or otherwise scary neighborhood, to remember that most of the people that live there feel trapped and would as soon be out of it as you, the cyclist. So a friendly wave now and then (using good judgment, of course!) might be a morale-builder all around.
I generally find people more accepting of this cyclist, in the bad neighborhoods, than in the fancy ones. Anything without at least four wheels, 300 hp and bulletproof glass is viewed with suspicion by the people that were scared to stay in the city in the first place, and fled (with their servants) to the outskirts.