I think this post belongs in the general structural discussion. Generally if a slab is supported by a good base layer, I would not include any weight of slab on the stem wall.
It can be affected by different compaction/fill properties under the stem wall versus under the slab.. or if anything was placed under the slab layer (e.g. void form) because of expansive soils. But in many cases, the difference is small enough that we can consider the load of or on a slab to carry straight into the subgrade material, and not load the stem wall.