Definitely not the case. In Salford, Cheshire, South Manchester etc you'd really struggle to find anyone who was a city fan. Source: I lived in Ashton-under-lyne from 1990-1995 or so. I personally knew literally hundreds of United fans (and this is before their really dominant era). During that time I knew literally 1 person who was a city fan and everyone would relentlessly take the piss out of him for it.