I would posit that balance is not the most important consideration. Accuracy and Truth are paramount.
Fact-checking is utterly and completely fundamental to any journalistic effort. Sadly, fact-checking takes time, money, patience, willpower and sometimes a fair amount of detective work. In today's fast-paced 24/7 news cycle, the temptation to rush things through, assume your sources are correct and save money, time and effort must be resisted. Take a look at this story on Media Matters about a conservative journalist getting basic facts wrong about the debate over 'balance' at the Corporation for Public Broadcasting. One word springs to my mind. Duh. In this article, basic fact-checking has taken a back seat to partisan advocacy.
Truth is much harder to arrive at. Ted Koppel correctly noted that there's a distinction between Accuracy and Truth; accuracy in facts can be verified relatively quickly -- truth is harder to arrive at and may take longer. (The relevant story about Koppel and Jon Stewart interviewing each other on Nightline can be found here.) Truth is harder to come by, often going to the heart, mind, soul and motivations of those people involved in the story. Regardless, hard labor in service of Accuracy will only serve the Truth, which is all any of us should serve -- journalists, politicians, parents, everyone.
When a news reporter goes to cover a story, their overriding concern should not necessarily be "am I hearing from all sides on this?" The reporter may well need to talk to many different people, gather many nuggets of information from multiple sources on all sides of an issue, but in the end some assessment must be made about what is really happening, regardless of what is being said on 'either side.' It's not about balance. It's about Accuracy and Truth.
You can read an outstanding speech by Bill Moyers about this and related topics here.