Preheating your oven is one of the first things you learn to do when roasting or baking anything. There is much debate about whether you need to preheat at all or if you should only do so for baked goods.
Preheating
Most recipes will tell you to preheat the oven. Cooking times mentioned in these recipes include the time to preheat the oven. In order to keep cooking times correct you should preheat your oven. This does not take long, about five to 10 minutes.
Do not preheat
The debate on whether to preheat an oven or not usually stems from energy usage while preheating. Some people argue that recipes were first created while wood-burning stoves were still in use and the preheat times were made for those stoves. According to Tree Hugger, preheating is wasting energy in your oven.
Considerations
According to Blue Flame, in a gas oven "preheating is usually not necessary for most casseroles, vegetables and meats" but should definitely be done for baked goods, like cookies, cakes and soufflés. Follow the manufacturer's instructions if in doubt.