As William Safire recounts in this week's "On Language" column in the New York Times Magazine, recently deceased author Saul Bellow once wrote, "Let the pure snows cool these overheated minds and dilute the toxins which have infected our judgments."
The grammarians are recoiling in horror by now--Bellow used "which" instead of "that" to introduce "have infected our judgments." This chump won a Nobel Prize? Were they just giving it away that year?
Of course not; Bellow is one of the 20th century's great writers. As he explained in justification, "'Which' sounded better than 'that,' and I do go by sounds as well as by grammar."
Great advice, Saul. I like to think of grammar as an envelope of suggestions rather than a tight box of persnickety rules. Too bad that I think of paper citation formats the same way!