I honestly think this "he supports...." thing that we hear on here about refs, journos etc is overplayed. Most of us are dyed-in-the-wool City fans, so we see things through that prism. If I was reffing a derby I'd think twice about giving a penalty against us, and I'm sure you would. But I think most people who make a living from the game don't have that fierce allegience that we have. You hear it from footballers all the time: "When I was a boy I supported...". I think once your inside the game that partisan thing fades, mostly.
But more importantly, as I said earlier, refs are professionals. It's their career. They feed their families on the salary and are extremely dedicated to what they do. And every ref at every game has an Assessor in the stand doing a report on his decision making. TV means that the assement is always bang on: wrong calls are always highlighted. So I don't believe that any of them would risk that career by showing bias towards or against any team. That's not to say that some aren't "homers": I've seen plenty of them. And I see them regularly making decisions to redress the balance of an earlier one. And of course I see loads of bad decisions.
But I honestly think that the sentiment that "he supports X therefore he is biased against Y" is in the mind of fans these days.