Bias matters not because liberals deliberately slant their stories, but because they are much more likely to interrogate the facts that contradict their ideological beliefs, than the ones that support them. When they come across an uncomfortable fact, they'll go out of their way to figure out why it isn't really true. When they come across a fact that confirms what they believe, they'll be more likely to accept it at face value.
I'm not claiming that liberals do this more than conservatives (I think that being human, they're equally prone to this phenomenon)--only that in the media, liberal bias is mostly what matters, because the media is overwhelmingly somewhere to the left of the American center. Even if you have a conservative reporter prone to insufficient interrogation of convenient facts, those same facts are going to set off alarm bells with his editors, who are quite likely to question the whole story.
This is why I think liberal media bias is worrisome--not because it's a vast conspiracy, but because it creates a giant store of pseudofacts that "everyone knows", like all those ludicrous statistics about abortion and domestic violence that used to appear everywhere. And new pseudofacts are being created, or resurrected, all the time. Longtime readers of the blog have endured my jeremiads against Republicans who say that cutting tax rates raises revenue. A few weeks ago I was flabbergasted to find out that a very smart left-of-center blogger I admire seemed to think it was common knowledge that there's really no evidence that tax cuts provide stimulus . . . a belief that I then find out seems to be common among some of the left commentariat. This is not at all what any mainstream economist I'm aware of believes (the debate is over the relative size of secondary and tertiary effects of various kinds of stimulus, not whether tax cuts are stimulative at all.) But somehow this belief had become common . . . presumably because in the milieu where it was transmitted, no one was disposed to question something that fit so neatly with what they already believed. Add in a few hundred repeitions from people you like, and voila, "Everyone knows . . . "
But we don't investigate things that everyone knows--reporters do not start off each new story by checking if gravity is still in operation. The more things that everyone knows, the more unnoticed holes there will be in stories. And there is no one without blind spots--the best you can achieve is getting together a bunch of conscientious people who all have different blind spots.