Energy usage of an oven
I will assume that your question is really "My oven is at 350 degrees, but I won't need it for another hour. Should I leave it on for the hour, or turn it off and then back on when I actually need it?". (This is more physics than cooking, but since you asked the question here, and I like physics...)

The answer is that you will use the least total energy if you turn your oven off, and then reheat it back to 350° at the last minute. Here's why.

If the oven temperature is 350° at the beginning and end of the hour, then the only thing the heating element needs to do during the hour is to replace the heat lost through the walls and insulation of the oven. The higher the oven temperature, the faster heat leaks, so having a lower average temperature (as it cools off during the hour) will reduce the heat leakage, and thus reduce the energy.

Calculating exactly how much energy you'd save by doing this would be pretty complex, although my guess is the answer would be "not all that much, electricity-cost-wise".

Edit (to add way too much information on this not-all-that-important topic, but as I said I like physics):

Think of your oven as a bucket with a small hole at the bottom. Fill the bucket with water (heat the oven) and the water (heat) will leak out the hole (insulation, door, etc). The deeper the water (higher the temperature), the faster the leak (rate of heat loss).

Say the bucket is full, and you need to have the bucket full at the end of one hour. Since you want to save water (heat/electricity), do you want to keep adding water to keep the bucket full (leave the oven on), or stop (turn the oven off) and refill it at the end of the hour (turn the oven back on)?

The key is the leaking water: whatever amount leaks is exactly what you'll have to put back into the bucket. So, the less that leaks, the less you have to put in. Since the lower the water level, the less that leaks per time; keeping the bucket full (leaving the oven on) will always leak more total water (heat/electricity) than if you let the bucket drain and then refill it (turn the oven off, and then on at the end of the hour).

The size of the leak (how good the insulation is), and the size of the bucket (how much heat mass the oven has) will affect how much water (electricity) you save, but in the end you will always save at least some water (electricity) by stopping and then refilling at the end of the hour (turning the oven off, and then on).

(OK, I'm done.)