Just to clarify, having a short torso or long legs has no effect on whether you may need a setback post or not, assuming you're talking about the same frame and same rider position. It's only if you have short or long femurs in relation to tibia length that setback is affected.

So the most important factor is the STA of the bike (because every 1 degree steeper needs about 10mm more setback to compensate). After that comes the rider's preference for where they want to be in relation to the KOPS "easy-to-measure" reference point. If you like being 20mm behind KOPS, like DaveSSS. then a setback post may be necessary even on a relatively slack 73 degree STA frame. But 20mm behind KOPS is quite a lot. I doubt most people are that far back.