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.