I never wai anyone. A smile and perhaps a gentle nod of the head is sufficient in my mind for me as I am not Thai. Been coming for 25 years and lived here for 12, but I am still not Thai and never will be.
The only time I can picture myself performing a wai is if I have an encounter with immigration officials or police where a positive outcome is dependant upon them, but that hasn't happened yet.
I cringe when I see farang at a bar, restaurant, or reception waiing the staff - eek!