James Harbeck gave a great talk on when to use “bad” English. Bad English in this case means nonstandard English: ungrammatical constructions, jargon, slang, and vulgarity. And the first lesson for editors is that you should use bad English when it’s not bad. Some of the rules taught in the past never had much basis in logic, grammar, or common usage, and are now widely dismissed as superstitions. So go ahead: split infinitives and end your sentences with prepositions.
James has posted a writeup of his talk on his blog Sesquiotica.