Well for one thing, it's as LandmarkWealth says.
Everyone should be looking over there shoulders, but not just because of black people, or specifically any race of people for that matter.
What happened to Trayvon Martin and Christopher Lane were both saddening trajedies.
The world is just overall a dangerous place, and will continue to get more dangerous as days continue to pass by.
Whether it's because people are desperate or just generally mentally unstable. People who kill others usually don't care for race. They're even willing to attack their own race.
However, I'm saddened that I have to admit that people will kill each other simply because their skin is a different tone or color.
People being mistreated because of color has been a recurring theme from since history began. And the only that has changed since then is the media coverage over the tragedy.
So finally, to answer your question, YES, white people should be looking over their shoulder, but so should every every other race on the face of the planet. And not because of a some tragedies that have recently occurred, but because the world is already dangerous place.