I don't know what kind of dishwashers you guys have. But I had a 15 year old basic Hotpoint and for 8 years I put dishes in it without pre-rinsing them. They often would sit for a few days before being washed too. And I almost never had a problem with them not getting clean. The few times I did, it was an obvious problem due to the object being blocked in some fashion from the spray. Two spoons resting againt each other, as an example. And of course there is an occasional baking pan or something that was heavily soiled that still has some material left, but that isn't a problem solved by pre-rinsing either.
Now, I have a new GE Profile. It's much quieter, easier to load, and holds a lot more stuff. Have had no problems with it either without pre-washing. And nothing in the manual says anything about having to run the hot water to make sure it;s hot before running the dishwasher. I have a fairly long run from the hot water heater to the dishwasher. And I fail to see how you;re saving any energy by running hot water down the sink to save "pre-heat." Even if it worked that way, you'd just be having the hot water heat it, instead of the dishwasher.
But the fact is, I don't think the dishwasher is heating the water during the first cycle or two anyway. When started, my GE Profile fills, washes a bit, and empties in just a couple of minutes for the first cycle. I think the design idea is that luke warm water is ok to start the first pass, before any detergent in even released. Houses are pretty much designed this way, so it seem logical to expect it to take a gallon or two of water before it gets really hot. In fact, the only cycle I'm sure that it does heat the water, is the last cycle. At that point, it sometimes just sits there, no washing for a min or two, while I'm assuming the water is heated to the final wash temp. Then it resumes and finishes the last cycle.