I think it's always helpful to know enough to answer recommendation questions. This includes stuff like:

  • What are the reputable brands?
  • What are the approximage price ranges for good pianos?
  • Where are the major dealers in your area?
  • Who in your area do you recommend for tuning and maintenance?

I also think it's good to know the major parts. Case, plate, frame, bridge, soundboard, keys, action, hammers, pins/pinblock, etc... Not really necessary, but it's nice to be able to answer questions from students about what such and such a part is called or "how does a piano work?"
"If we continually try to force a child to do what he is afraid to do, he will become more timid, and will use his brains and energy, not to explore the unknown, but to find ways to avoid the pressures we put on him." (John Holt)