You can preheat the toaster oven if you want too and it's highly recommended that you do preheat your toaster oven before placing the food in the toaster oven to cook.
The reason you should preheat your toaster oven is to allow the food to start cooking faster since the toaster oven would already be hot and not cold.
However you do not have to preheat a toaster oven before cooking food in them if you do not want too.
But while I prepare my food to be put into the toaster oven I go ahead and get the toaster oven preheated.
That way the toaster oven is warmed up to the desired temperature and the food begins cooking immediately without having it starting from cold.