The key to all good writing is understanding your audience. Every time you use language, you engage in a rhetorical activity, and your attention should always be on the effect it will have on your audience.

Think of grammar and style as analogous to, say, table manners. Grammatical "rules" have no absolute, independent existence; there is no Grammar Corps to track you down for using "whose" when "of which" is more proper, just as Miss Manners employs no shock troops to massacre people who eat their salads with fish forks. You can argue, of course, that the other fork works just as well (or even better), but both the fork and the usage are entirely arbitrary and conventional. Your job as a writer is to have certain effects on your readers, readers who are continuously judging you, consciously or unconsciously. If you want to have the greatest effect, you'll adjust your style to suit the audience, however arbitrary its expectations.

A better analogue might be clothing. A brief submitted to a court calls for the rough equivalent of business attire. However useless or ridiculous a tie for men may be, however outdated its practical value as a garment, certain social situations demand it. And if men go into a job interview wearing a T-shirt and jeans, they only hurt themselves by arguing that the necktie has no sartorial validity. Your job is to figure out what your audience expects. Likewise, if your audience wants you to avoid ending your sentences with prepositions, no amount of argument over historical validity will help.

But just as you shouldn't go under-dressed to a job interview, you shouldn't over-dress either. A white tie and tails will make you look ridiculous at a barbecue, and a pedantic insistence on grammatical bugbears will only lessen your audience's respect for you. There are occasions when ain't is more suitable than is not, and the careful writer will take the time to discover which is the more appropriate.
Guide to Grammar and Style by Jack Lynch.