What is the appropriate fuel tank selection to land a 172?
You should check your POH but it may just be that the instructor in question was recently flying a lot of Pipers and it was force of [bad] habit.

This POH for the 172S is in agreement with you. 


  BEFORE LANDING 
  
  ....
  
  
  Fuel Selector Valve -- BOTH.
  


You should consult the checklist in the POH that is with the airplane in question to be sure.