Most foods with shorter expiration dates (milk, bread, cereal, etc.) are usually good beyond their exp. dates for 7-10 days. It's just the manufacturer can't guarantee freshness beyond a certain date. YOUR mileage will vary.
Other things like condiments (Mustard, catsup, mayo, etc.) have a relatively long exp. date. You want to toss it when it says its expired......probably even when it's just close to the exp. date.
But a general rule of thumb IS to toss it when it's expired. I think most people can judge for themselves when something isn't fit to eat anymore and will toss it instead of eating it. Some things are fine past their exp. date, it's a judgment call.
I float the exp. date, but not to any kind of extreme.
.....and I very rarely ever get so much as a cold. And when I do, I blow through them within a day or two. No kidding.