You clean under it so that the cut stuff can circulate around the blade(s). Over time the stuff builds up more and more and the clippings only get cut once. This is a bigger deal on, say, a mulching finish deck than it would be on a brush hog.
And I've found that the only way you can really get a deck clean is to turn it over and scrape away at the spots that won't come off with some kind of hand tool that is meant for a completely different purpose, bark your knuckes against the blades, start bleeding, remember that gloves are a good thing, and then start either cursing, crying, or both /forums/images/graemlins/grin.gif.
Just a little joke - although this does represent real experience /forums/images/graemlins/blush.gif. WD-40 doesn't hurt, but I've tried it and am not convinced it really helps all that much. I've waxed the deck, and that helped some, but only for a couple of "mows" and it isn't easy to do. Haven't tried Bird's method, but it sounds good - except I don't always have time to really clean my deck after each mowing /forums/images/graemlins/frown.gif.