For cartoon animation exaggeration is common. This is one of the uses for stretching. Even in realistic animation, sometimes you need to stretch the body or limbs to get a realistic feel to the animation. It’s one of those things that comes with experience. As for your second question, Yes it is common to have switch property to turn stretching off/on. Some riggers do all stretching. Others do individual zones like legs, arms, etc. It just depends on script and the demands of the animators.
