Your main goal online is to win. If you're constantly changing cars, instead of using the ones that you are the best/most comfortable with, it's just going to make it harder for you. It's harder to keep track online as well. You'll have to check which ones you haven't driven in your stats between races. And the discipline/car class is pretty random online, so you won't always be able to pick a car you haven't driven yet.
And what would you be saving your cash for? The only thing to buy is cars, plus the required upgrades for some events. And like I said, you have more than enough money to do this. I have 4 mil leftover with all cars purchased after completing all 100 events doing a mix of easy, casual, and serious difficulty.
The reward cars aren't necessarily the best. I had no problems winning any races with any car I used. Especially when the difficulty was set low. I only found 2-3 of them useful. And even then I wouldn't have said that they were extremely beneficial for me.