But—for the millionth time!—an opinion is not a bias! The fact that reporters tend to be liberal says nothing one way or another about their tendency to be biased. It does suggest that when political bias does creep in, it is more likely to tilt liberal than conservative. But there are so many other pressures and prejudices built into the news—including occasional overcompensation for fear of appearing biased—that raw political bias plays a fairly small role. And any liberal bias in reporting is more than counterbalanced by the conservative tilt of the commentariat. Or so I believe.
Of course it is not easy to persuade folks of this, and many will never believe it. No doubt it is easier just to keep your political opinions secret and imply that you don't have any. But that absurdity or dishonesty itself undermines your credibility. Or it ought to.