ABGD, I reckon that you're right to leave nav until you have some practical experience under your belt. Most of the others can (and probably should) be done in advance; the thought of learning to fly during the day and then cramming for exams during the evening doesn't sound like my idea of fun. My kind of down-time is far more Homer Simpson-like.
As for schools that want to charge you more for having done some of the exams in advance, my interpretation of that is that they are trying (in not so many words) to tell you what kind of people they are, and what it will be like dealing with them when you get there....
