Primarily commercial photography (95%), self taught in photography, but had a business background from working in the corporate world for a number of years (I got to see how you shouldn't do it
). I never assisted, but am a sponge and soak up things very well on my own. I think I literally have every photography book published that has anything to do with photography techniques, or the business, legal issues, and promotion or photography. I'm also pretty active in the professional circles and try to stay abreast of what the industry is all about. I don't read typical photography magazines, but gravitate to those that are more industry based (PDN, etc). I also shot actively for my own enjoyment for 15 years before deciding to do it as a business.
I meet so many students each year that approach me to assist, and find they know very little of what's really needed in my opinion to be ready. I've found most of the more than competent with a camera, but lack any understanding of the industry, pricing, usage, organizational structure of the clients, basic clients needs, preparing files for press, etc. If schools taught what they do now, than took it another 6 months to add in business, industry trends, pricing/bids/estiamtes, usage, and so on, I would absolutely recommend a school first.
I believe that if you are a self starter, motivated, and with the discipline to approach this as a business, and not a hobby, it can be done without schooling. But with the extreme competitiveness of the industry, if you don't approach it as a business, regardless of your talents and photography skills, its going to eat you up and spit you out. There is just so much to learn past the f-stops and composition, creativity and vision, that the photographic part is only a third of what you must really know well to succeed.
Probably the best recommendation I would give would be a three prong approach. Some school on the techniques of photography to get the ball rolling. Then self study, workshops and seminars, and assisting for an established pro. And finally some small business courses which include accounting, cash flow, marketing research, administration, and federal/state/local regulations, and some self study on the photographic industry.