Most of the time the "best by" dates are statistical points of when it should last be sold -- anticipating that it will sit around substantially after that in your kitchen, unopened then opened (time depending on the product). I've had years-old salad dressings be no worse than bland.
On the other hand, buying plain yoghurt lately drives me crazy: you pretty much can't buy a 1-cup portion anymore: the big guns such as Dannon are all 6oz (they reduced size when sugar first got expensive, but there's no sugar in the plain yoghurt), even some of the higher-end brands come in 7oz. So I end up buying a quart.
My general experience on plain yoghurt is that it will remain unspoiled for two days less than the second time you need it, at which point it turns blue and fuzzy.
What is patriotism, but the love of good things we ate in our childhood?
-- Lin Yutang