We will place the bathtub on the screed. There are mainly 2 reasons for this:
1) Underfloor heating can be laid through
2) It provides more flexibility for future changes
There was actually no reason for us to place it on the raw floor instead. I personally find getting in this way even more comfortable, as there is no difference in level.
However, from past experience, I only know it as placing the bathtub in a recess of the screed, so this is by no means a new development. I suspect the reason for this was the otherwise occurring point load on the screed, which today is countered by using a bathtub shell made of EPS.