Well, some scenes can be close to 256, but that is far from the majority with my GTX 960 where it generally works best with large sizes. And in this other user with GTX 1080 Ti also most of the times are sizes greater than 256:
https://blenderartists.org/t/gtx-1080-ti-various-benchmarks-bmw-classroom-fishy-cat/685785/44
https://blenderartists.org/t/gtx-1080-ti-various-benchmarks-bmw-classroom-fishy-cat/685785/48
Anyway the headaches to configure optimal tiles will end with Blender 2.8 (hopefully)