Obviously, different people are going to find different levels of conditioning acceptable depending on exactly what and how much you intend to break.
At our school, breaking is required for all promotions. All the required breaks are kicks, but Dan ranks are allowed to choose their own breaks and anything goes (I've done various hand techniques, elbows, knees, and head breaks in addition to the traditional kicks).
Required breaks are all 1" pine, but people do choose to do more. With proper technique, an injury is unlikely with a 1" pine board, even for the smaller students.
I think that although conditioning is important, it's not something to spend a lot of class time on. In class, we try to teach good technique. Practicing those techniques outside of class, on a bag or other target, provides conditioning.
What kind of conditioning do you practice and teach?
Personally, I condition by beating the crap out of BOB. The BOB is much firmer than other freestanding bags I've used (such as the Wavemaster). I used to use bag gloves, then switched to regular gloves, then hand wraps, and now bare handed. I also use a makiwara, but honestly I am not convinced that it's any more effective than the BOB. For spearhands, I use a 5 gallon bucket filled with sand.
Students are told to use bag gloves and gradually increase the power of their strikes. We don't teach spearhand conditioning.