Much has to do with the specific IR Emitters and Detectors being used. Typically, you want them facing forward, but angled out slightly. You will have to play around a bit... the ideal set up is one where only the left sensor picks up objects on the left that could hit the robot, only the right sensor picks up objects on the right that could hit the robot, and both sensors would both pick up objects that are directly in front of the robot.
Please note that IR sensors aren't good at shape recognition, they can't distinguish between a red ball and a blue box... they can only detect if something is in the path of the IR light being emitted.