Many airfoils still provide lift at negetive angles of attack with respect to the airflow. Depending on the weight and the speed the model is designed to fly, it is not unusual for a main wing to fly at negetive incidence.

Incidence shown on a plan however is a different story. It is relative to an arbitrary 'datum line' chosen by the designer by which the wing, stabilizer, and thrust angles are compared. So you could have negetive incidence of two degrees on the wing, stabilizer, and thrust line reletive to the 'datum line' but yet they would all be parallel to each other. So when you look at the big picture of an airplane design you need to take into account the incidences of all the components and how they relate to each other via the datum line.