Superstitions about baking and roasting account for much energy wastage in the modern kitchen. "Preheat your oven" is an old wives tale that, with perhaps a few exceptions, can be ignored in the interest of energy saving.
When European and American cooking tools and classic recipes were developed, wood- or coal-fired home ovens were slow to come up to temperature and ovens were unevenly heated until the cook had time to spread the embers and wait for heat to disperse from all sides: hence, preheating made sense to our ancestors and early cookbook authors. (I make this assertion having baked and roasted in wood fired ovens as well as in various modern gas and electric models.) With modern electrical or natural gas ovens, especially the smaller volumed ones - preheating is a cook-time saver but otherwise is little more than an energy waste, so much the worse if food preparation ends up taking longer than you had estimated while the oven "preheats". Yet, recipe books all call for preheating. Worse, parents continue to teach their children to follow the practice without thought of the energy consequences.
Conceivably, the baking of elegant soufflés or such may benefit from preheating the oven. Just maybe. Otherwise it's largely bunk and especially pointless for roasting of meats. We wish the Myth Busters would have at this issue; but, it's probably not macho enough a subject for them to tackle. Logic will have to suffice.
Think about how much energy is wasted if you bring the oven up to 425 degrees Fahrenheit for half hour before baking begins, then open the oven door all the way to insert the pans. It's every bit as crazy as opening the door multiple times to "peek" at the results while the food bakes or roasts. (If you must peek, clean the glass and look through it. Leave the door closed.)
It's true that you may "save" a few minutes bake/roast time by preheating, but the trade-off is energy expenditure. Conversely, by not preheating, you won't have to wear oven mitts while loading in your pans, reducing the risk of burns to you and the climate.
Equally wasteful is the practice of leaving the oven on "the proper setting" until the end of the specified cook time. A hot oven with the door kept closed loses heat quite slowly. Generally, I shut mine off and let it "coast" to the finish at least 15 minutes ahead of the time I expect to remove the baked food. For a large roast or "baked dish" I might shut it down 40 minutes ahead of time.
The worst thing that can happen from letting the oven "coast down" is you may occasionally have to turn it back on for a few minutes at the end. Try it. You'll be surprised how well residual heat works.
The final myth is a more modern one that relates only to electrical ovens.
Important caveat:- Gas-fired ovens need to be kept clean. Ash and 'crud' from boil-overs or spills insulates and slows heat transfer to the oven's interior. Keeping a gas oven clean actually improves energy efficiency by reducing the time needed to come up to temperature, thus mitigating against the need to pre-heat.
An electrically heated oven has the heating elements inside the cooking space. Cleaning the interior surface of an electrical oven, especially one with the "self cleaning" feature, does nothing to improve energy efficiency of the cooking process and in fact accomplishes the reverse by consuming large amounts of electricity for what amounts to a purely aesthetic intent. (Very likely the cultural root of that intent is to accomplish what was once needed for gas and wood stoves.)
To summarize, there is an efficiency reason to clean a gas oven but no such thing for an electrical one. It all comes down to aesthetics.
Image credit:Vintage Stoves Oven Door Open