I should stress that I don't teach people to fixate inside. In the circuit the only instruments I get students to glance at are the altimeter and ASI (also tacho- although I hate it when people try to set power to the individual rpm, I think it takes time to get accustomed to how changes in throttle will affect rpm, so it needs to be checked). I must say, though, I have seen a lot of people get dangerously slow when they pay no attention to their airspeed on final, so if I were to start covering instruments I would wait for a smooth day.