if your car is stock, then i guess its fine to follow as the manual says.But if you have upgraded your engine and components, its a different thing right? since the air flow has increased it requires more fuel to cmplete the combustion,if a lower grade of petrol is used,it can cause knocking,if its severe it can lead up to detonation.(sifus please correct me if im wrong).
and for those who have already tuned their cars with piggybacks,wouldnt using a different grade of octane cause severe changes in readings?