I worry less about expiration dates with yogurt than with some other things (like hummus, which seems to go bad really fast, for example). If it's live yogurt, I figure the good bacteria help keep other (bad) things in check. Usually when you leave it past the sell-by date it gets a little tarter-tasting, but that's all -- until it looks or smells bad.
I don't always just go by smell though -- ESPECIALLY with leftovers. If something has been sealed in a store-bought container, and looks and smells fine, it probably is. But the same doesn't apply to stuff cooked at home, served, cooled, transferred to another dish, refrigerated for several days.... I always heat leftovers thoroughly if they're more than a day old.
Sorry, got a bit off-topic!