I love adding niches to shower areas, for the practical storage capabilities and the beautiful accent. For a tub they make sense just above the tub surface, as you’ll be lying down when you reach for that shampoo, but in a shower they should be much higher, around 48 in., so you don’t have to lean down to reach anything.
Place your shower controls around 42 to 48 in. at the center, and a tub filler 4 to 6 in. above the top of the tub.