Cooking at home as much as I do, it seems to me that if, at the point that my hunger was satisfied, I still had "extra" food on my plate, then I cooked too much for that one meal.
Don't get me wrong--it's always good to stop eating when your hunger is satisfied. And there are circumstances where cooking less is not possible--you can't exactly tell a restaurant to cut back on the amount they make. It's generally one size for one price, and I suppose you could always "doggy bag" whatever's left over.
But at home, I prefer to get what I pay for and that's why leaving food on my plate never enters into the equation. If I can save it for leftovers, fine--I always have leftovers from frozen 12" pizzas and Hamburger/Chicken/Tuna Helpers--but there is no way anything gets left on my plate. My retort to that age-old saying about leaving a little on your plate is this: it would be less wasteful to cook less food than to leave "extra" food on the plate. A few less rotini in the boiling water here and a few less tater tots in the oven there would surely stretch your food dollar farther.
Or better yet, buy a little less food to begin with!