I teach on DA20 Katanas and in a PA28. I also teach Tailwheel in a SuperCub and aeros in a T67.
It is possible to teach ab initio in a SuperCub, but in my opinion, not a good idea. It is surely much better for the instructor to be able to reach all the controls (especially the mixture knob!), tune the radio, read the instruments and see where the student is looking or what he is actually doing with his hands and feet. Side by side is a far superior format for teaching. It will also take some students much longer to get to solo standard in a tail dragger. For some, the frustration would be too much and they would give up.
Instructing on tailwheel aircraft would probably produce students who pass their test with higher levels of handling skill, but there would be fewer of them.