Personaly I would become an independant agent reping mulitiple companies other than SF, especially if you are just starting fresh without a book of business. The reason I say this is because if I am not mistaken SF commision rates are not the highest (8%-Auto, 10%-home).
I know that if you are appointed with a company like Travelers you can expect 11% for auto and 20% for home. They also pay 15% for commercial (not sure about SF).
The point I am trying to make is that YES, SF is like the Mc Donalds of ins and you will have corporate franchise suport, but you will have to work very hard if you are starting a scratch agency. Dont get me wrong, you will work hard no matter who you rep, but I personaly would rather work just as hard and know I am making a higher commision rate reping companies that are just as well known as SF.
I personaly am with an agency that is appointed with 20 plus companies. I will not say that I always beat SF rates but we are very competitive. SF is a great company and does do well. Just make sure you weight out all your options. Afterall I know that for your first year you dont really have a contract with them and they can terminate you at anytime if you are not meeting your quota and they also will take over your location from you.
Point being = weight every option and make sure you have all your ducks in order ( especially if you are starting from scratch)
Just my 2 cents!