Avoid pre-heating your oven

Ignore cookbooks! It is usually unnecessary to pre-heat your oven before cooking, except when baking bread or pastries. Just turn on the oven at the same time you put the dish in. During cooking, rather than opening the oven door to check on your food, just look at it through the oven window. Why? Opening the oven door results in a significant loss of energy. [Who says so?]