It depends on what model is and what its purpose is, it also depeneds on what artist is trying to achive. For example if the model is meant for real time render (games etc) than in order to save memory all corresponding objects should be unwrapped onto a single texture in order to save memory. Obviously if you bake everything onto a single texture you will lose quality wich isn't good, for lets say, HQ static renders etc.. There is so much varation that depends on a milion things, general rule doesn't exist. You do stuff in a most efficient way posible keeping the quality high. Some models are done "by the ear" some are limited by performance of the render, and some are done in "sky is the limit" fashion.