I have NO idea who started that rumour... but it's perpetuated by forums and magazines and blokes in garages across the country.
No such rule exists!
Apparently the law that DOES exist is that the biggest you can go is what was offered on the car (as in, you can buy a VY with steelies and stick 18" rims on it, because the SS had them, and that's what that model of car has been engineered for.)
But I'm yet to see any evidence of EITHER laws.
Either way, I have 15's, I know I'm safe
edit: Oh let's not forget that 16's were available on VKs - 95% sure the HDT rims (forget their name... gr!) were available as a 16... so even going by the first law and second law 18s on a VK is legal