It mostly likely depends on what the nature of the training is. If its a super basic intro to blender and a guide to its user interface that, I expect, would probably need a 2.8 version once its available. If its instruction on mesh modeling or sculpting then I think the vast majority of it will still apply to 2.8
Of course who knows when 2.8 will be available and delaying content until a future release can mean waiting a lot longer than expected and in the meantime people are still interested in learning 2.79.
It might depend also on if you are making like “the definitive blender course” that you don’t want to have to update for a long time, or if you are making “how to model a house in blender” that you would probably do another version of in a year or two anyway.