For many, it's a common sense thing, based on the name of the appliance itself. But those who do scrub down dishes before loading them in the dishwasher waste water and electricity, and may harm their more delicate items.
Even if you think having cleaner dishes in the dishwasher might make the device run more efficiently and give you spic-and-span results, you're still wasting water, electricity, and detergent, as one appliance design consultant explains. Worst of all, one "senior dishwasher design engineer" (and, hey, who better to turn to?) says trying to beat your diswasher at its own game can lead to damage:
"Dishwasher detergent aggressively goes after food," Mr. Edwards said, "and if you don't have food soil in the unit, it attacks the glasses, and they get cloudy," a process known as etching that can cause permanent damage.
The Q&A article this advice comes from has (no pun intended) loads more advice on how to get a better, more efficient clean from your dishwasher. Never load bowls upright, for example, because they'll carry dirty water between loads, and, in most cases, you don't need heat drying or the pots & pans cycle. Why isn't my dishwasher cleaning my dishes? [New York Times via Consumerist]