Preheating a baking sheet before putting in cookie dough is not typically necessary, although it can have certain advantages depending on the desired outcome.
Preheating a baking sheet can promote faster and more even baking by providing an immediate burst of heat to the bottom of the cookies. This is particularly useful when you want your cookies to spread and brown evenly. Preheating can also result in slightly crisper cookies since they start baking from the moment they touch the hot surface.
However, if you prefer cookies with a softer and chewier texture, you may want to skip preheating the baking sheet. A cooler surface will slow down the initial melting of the fat in the dough, which can lead to less spreading and a thicker, more tender cookie.
In general, the decision to preheat your baking sheet depends on your desired cookie texture and appearance. If you're uncertain or following a recipe, it's a good idea to refer to the specific instructions provided.