Pilot DAR
The reason why I believe that some upper air work is necessary is straight forward enough.
If a student is undergoing a tailwheel conversion then by definition they will not have piloted that type of aircraft previously. Therefore they will not be familiar with the individual handling characteristics of the aircraft and as such some familiarisation of steep turns and stalls would be beneficial. I think you would agree that in the early stages of learning to land a tailwheel aircraft, mistakes will be made and the old adage of ' if in doubt, go around' is vital. As such some practice of go around handling in various configurations will give both student and instructor confidence before going on to the process of landing the aircraft whichever technique is taught first.
Regarding the sideslip, I have found that it is a technique that is not taught well at PPL level neither is it practiced. It is quite possible to mishandle a sideslip approach, particularly the transition phase, so why not give the student the opportunity to look at the manoeuvre with some altitude to play with below.
I thoroughly agree that the Wheeler landing is under-rated as a technique - some tail wheel pilots are obsessed with the three-pointer despite all the variables that may make it very much the less favourable option.
Obviously every student is different both in terms of experience and capability. It is our job as instructors to formulate a training plan accordingly which covers all the relevant aspects of tailwheel training.