Accept any mode of communication as valid, as long as your communication partner understands what it means. Don't require individuals to repeat themselves in another modality. Do model the response in the modality you are trying to teach.
Supporting Research
Binger , C. , Berens , J. , Kent-Walsh , J. , & Taylor , S . (2008) . The effects of aided AAC interventions on AAC use, speech, and symbolic gestures . Seminars in Speech and Language, 29, 101 – 111