Go ahead and B-spec for cash, but only once you are tired and bored of A-speccing the same race for cash. After all, you need A LOT of cash to do all the silly one-brand races that give you little money and garbage cars as prizes.
The best places to make cash are:
Costa D'Amalfi rally (A-spec, before 20% completion).
200 mile El Capitan endurance (B-spec, after 20% completion).
Deutche Touring Mastersheife - probably mispelled (A or B-spec, after you get IA license).
A suggestion... build up your B-spec driver stats, you will need it later. I did it the easy and fun way, by B-speccing every championship right after A-speccing it. I also suggest using the lamest car that you can possibly use and still win. That way you will be forced to actively manage your driver to win, making the process enjoyable rather than a time to go away for coffee.
Above all, have fun. Remember, you bought this game to have a good time, not to go through "I have to complete this" torture.