A moderate degree of head tilt, that which may occur in natural free head motion, is acceptable and does not affect performance. This provides you with the flexibility to let people act naturally, moving their head as they normally would when looking at a computer screen. The result will be a more accurate analysis of real-world environments.
The video below shows a quick demonstration of an eye tracking study focused on head tilt. You can see the eye movements represented by red lines and the spots where the subject hovers represented by the red circles. In the right-hand side is the recording of the subject looking at the screen. You can see the subject tilt his head from side to side; the calibration of the system remains as does the identification of the eyes so that tracking is continuous.