Hi Gibby;
Whenever I check an Oven Temp. I wait until the oven reaches the Setting.
Then I'll check it about 5 or 8 minutes after that, so that it is constant.
If you have an Electric Stove, don't check it while the Burners are Red,
because it is bringing the Temp. up, after the Oven has cooled,
so it may be hotter at the top shelf of the oven.
Wait until the burner shuts down for a minute.( not Red )

If you have a Gas Range; same thing,
wait till it reaches the Setting and
the burners go off or the flame goes lower.

Make sure you thermometer is accurate. Check it with
another, that you know is right.
Hope this helps,
Lou

It's called the "swing factor." Thermostats allow several degrees "swing" up and down from the set temp to trigger the on-off cycling. You can adjust the swing factor on your furnace thermostat by two to three degrees each way. Since oven temps are so much higher than home temps, I'd guess the swing factor would be much greater in an oven. It's that way for all appliances.

Electric appliances (and furnaces) have a smaller swing factor than gas. That's why, if you bake more than you roast, it's better to have an electric oven so you get more consistent temperatures and a better baked product.

WHen an oven heats up, at first you are heating the air inside the cavity, and then when you open the door, the hot air escapes. For a more stable oven temp, allow the oven to heat for 10 minutes past the pre-heat 'beep', and then the oven itself will continue to heat (walls, floor, etc) and the heat loss when you open the door will not be so significant.

You never need to preheat when roasting or braising meats or baking potatoes. Start the oven when you put the food inside, save the gas or electricity. You MUST preheat when you brown a whole filet mignon or a steak on the stovetop and finish it in the oven, or of course when baking breads and pastries.

Sushipup, I've been thinking about your post. I'm not sure how that works, since ovens are very well-insulated so that the heat remains inside. Can you explain a little? Ten minutes extra seems like a very long time, wasting perhaps 10 to 20 minutes of fuel and time. Does a long preheat give a far superior result to baked goods? How can I tell?

I forgot to mention broiling. Never preheat to broil, either. I googled preheating; many sources say preheating is a wasteful throwback to wood fires, that even for baking no preheating is necessary. I'm not sure I want to risk my next cake to a cold oven, though.

Simple, open the door, heat escapes. More heated mass means that less heat escapes, and recovers faster. That's why many balers use bricks or even brick ovens,

Also, since ovens are quite well insulated, most of the energy is used to get the oven up to temperature and some more to recover for air exchange when the door is opened. The energy used to keep the oven on for 10 minutes more is pretty negligible. I looked into energy costs of running ovens and range top burners when I was buying and it comes to pennies.

If something is going to be cooking a long time and you aren't trying to get some instant effect (forming a crust) when you first put it in, then preheating fully probably isn't important. If you are cooking things that take a short time especially where you want heat to do something to the outside vs the inside, like getting a crisp crust on oven fried potatoes or pizza, then it is important. Also for getting thin breads like pita too crisp or for giving the bubbles the best chance to stay in for baking soda based baked goods. For roasting meats, I like the crisp crust by starting with a very hot oven and then dropping to a lower cooking temp so preheating would be important there too.

Sherrmann:
That doesn't always hold true about Electric being better for Baking.

I built my Pizza Oven . It had stone Shelves and was Gas.
It would have never made it, if it was Electric.

The recovery would not have been fast enough.
Of course there are Electric Pizza Ovens.

Mine was an exception. Might not hold true for all ovens.
Lou

Ha-ha - you all have such sophisticated oven needs compared to mine!! I wasn't sure if I should ask about this in appliances or over here but I figured you guys would really know about ovens and I was right!

I think it does heat up a bit beyond the set temp to allow for opening the oven and putting cold/room temp items in. In any case it pretty quickly gets to the right temp and stays there so that's really all I need.