When are words dirty?

Wallace Stegner arrives at an astute definition, and it was back in the 60's, but still rings true.

Words are not obscene: naming things is a legitimate verbal act. And "frank" does not mean "vulgar," any more than "improper" means "dirty." What vulgar does mean is "common"; what improper means is "unsuitable." Under the right circumstances, any word is proper. But when any sort of word, especially a word hitherto taboo and therefore noticeable, is scattered across a page like chocolate chips through a tollhouse cookie, a real impropriety occurs. The sin is not the use of an "obscene" word; it is the use of a loaded word in the wrong place or in the wrong quantity. It is the sin of false emphasis, which is not a moral but a literary lapse, related to sentimentality. It is the sin of advertisers who so plaster a highway with neon signs that you can't find the bar or liquor store you're looking for. Like any excess, it quickly becomes comic …