Stereotypes are best avoided altogether, unless you're writing satire, and profanity and slang are best used sparingly. With regard to stereotypes, only write in dialect if you know the culture intimately: Any Southerner will cringe if a character says "Pa" in this day and age, and unless you grew up in Brooklyn, think twice before inserting, "Fugeddaboutit," into the mouth of your Brooklyn cop. Get it wrong, and you risk appearing naïve and/or offending your reader.
As far as profanity and slang go, both will quickly date your work -- rather than make your characters look cool or tough. Hemingway, whose characters included soldiers, fishermen, hunters, and artists, had excellent advice on this subject: "Try and write straight English; never using slang except in dialogue and then only when unavoidable . . . slang goes sour in a short time." And you'll notice that even with his toughest characters, profanity is as rare as slang.
Return to "Top Tips for Writing Dialogue."