Of course, it also depends on what you call "media coverage" (CNN vs NY Times), whether you include editorials or not, and what time period of the Clinton administration that you are looking at.
Its easy to manipulate the data based upon what criteria you are looking at.
Generally, the research shows that self-censorship occurs for the two reasons above: A.) For money, or B.) in order to protect whoever is in power as to not alienate them as a source.
As they get tons of news from the Administration and take their word on it for a multitude of reasons (goodbye investigative journalism), this causes them to invoke in reason B.
Reason A occurs a helluva lot more in the that they tend to not publish negative information about their advertisers, supporters, the media, and company interests.
When the "scandal" thats selling newspapers (such as the whole Clinton blow job bull sh-- or Whitewater) is from that source, they don't censor themselves and start being attack dogs.
Its a balance for them between self-censorship and printing stuff that is negative toward the person in power (aka one fo their major sources).
Case in point: NY Times' news stories in the lead up to the Iraq war.
The reason why I say that the majority of Clinton news coverage was negative was that the media was playing as the attack dogs in this one by helping the Republicans drum up a scandal that shouldn't have been news in order to sell papers.