The Bolton v United rivalry is geographical, isn't it? The western side of Manchester, and going beyond the city boundary, Salford and Trafford Park have always been United strongholds. As you head north of Salford, you move towards Bolton, and there are districts that are inbetween the two, like Swinton, Clifton and Pendlebury.
(My old man - a big blue - lived in Clifton for a while when he was growing up, but had lived all his early years in Moss Side, hence the City allegiance. I don't know the area, though, so someone who does please correct me if I'm talking sh!te).
Anyway, as I've always understood it, the rivalry between Bolton and United grew up because there were areas between Salford and Bolton where both teams had a lot of fans. Bolton had less of a rivalry with us because our core support base tended to be in east Manchester, further re oved from Bolton.