Enabling grid lines in the default Camera app can help make sure your photo is not only straight, but that you're using the rule of thirds in order to capture the most interesting photo you can. In iOS 7, the way you turn grid lines on and off in the Camera app has changed slightly. If you're fumbling around trying to figure out how to turn them on, follow along and we'll help.
That's all there is to it. Grid lines should now be activated inside the native Camera app.