My Dw can use high termperatures for washing and rinsing but I still like the idea of putting a little bit of detergent in the prewash cup when using powders or tabs especially since the first fill on my machine is mostly cool water. My prewash cup only holds 1/2 tsp or 1 tsp or some small amount like that. The prewash for most cycles last about 13 minutes, but some of the stronger cycles have the prewash lasting for 25 minutes and during these the water will be heated to steaming so I always use detergent in the prewash for these cycles. There are also the cycles that don't have a prewash at all and just a wash....two rinses and dry ....these work very good too but after using them repeatedly for a week I will follow up with a cycle with a prewash using a small amount of detergent in it.
Like Joes DW, mine will skip the water change between prewash and wash for lightly soiled dishes or small loads but most of my loads are heavy so this rarely happens and it is more likely to insert an additional after rinse than skip the prewash water change.