I took an aerobics class in college and was surprised to hear the teacher say, "you don't need to do crunches, they are unnecessary. You work your abs with every movement you make, all day long, and in every other exercise." She is partially right. Most of us use our abs all the time, but that is exactly why we need to strengthen them. That is your core that keeps your back straight. If nothing else, practice sitting up straight, all the time - here at your computer, when on the floor, changing a diaper, anything you do. It will help develop those muscles and keep you from getting a hump on your back.
The other point here, as the title suggests, is that you should really look at people when they give you advice. Just as you wouldn't want to follow a financial consultant who is broke, you may not want to take workout advice from someone who does not have the kind of body you enjoy, like my aerobics instructor, who, while being in fairly good shape, did not have the sort of belly that I would enjoy.
A more positive way to say it is this: when I was on drill team in high school, we danced pretty hard for at least two hours a day. I practiced even moreso on my own, hoping to make the cut for each dance, as did many of the girls on the team. By the end of the year, all of us looked roughly the same. So pick your model, and do what they do, unless it involves anorexia, of course.
