Point-of-view filters

What disturbs me sometimes about biographies is that our ideas of a person are ruled very much by who is still around and willing to talk (or whose words are still around) when the bio is being written. You can't always tell who has an ax to grind, or who has an interest in sugar-coating a reputation. This is not to mention the agenda of the biographer.

In fiction, we can play with these ideas, controlling these points of view deliberately. The character who badmouths someone else: is she providing fair warning, or unjustly smearing that person? Is the narrator being honest? And just how rose-colored are the lenses through which we see the love interest?