Heptane is slightly less flammable than iso-octane but I doubt that plays a big role. It might just be one of those old standards that is still around because that is how it has always been done. It is also a tough decision to make if you are going to get into calibrating injectors as pump fuels have such a wide range of organics present. Most gas in the US has ethanol in it so do you test with that even though it slightly denser than iso-octane? When ID says they test with gasoline do they mean pump gas with ethanol, pure iso-octane or racegas? Pump gas is not even pure iso-octane it is a broad distribution of organic compounds depending on the reforming process and source of the precursor hydrocarbon.
Heptane is also a much longer molecule so its rheology will be different than iso-octane, and both will differ slightly than commercial pump gasoline. The substance known as gasoline can vary so much in composition it is almost worthless as a standard since each batch will vary slightly.