So for what it's worth, I've been playing for two days and have a garage value of $30,000,000+. I don't add more than $500K, 2,000 gold, 2,000 bronze keys, 1,000 silver keys, and 500 gold keys at a time. I do this maybe three times a day (the gold key collections are what drove my garage value so high).
I have yet to be banned from doing this, but again it may take time for a ban to come through. I've already beat the game (and gave $3 purchases for the super nitrous to unlock the boss cars, this is so I don't feel too guilty about having fun for hours without giving a little back to the developers lol), so I'm not terribly concerned if I get banned at this point.
Anyway, take it as you will, just figured I'd give a bit of info on my experience.
edit: oh, also, I use LOTS of gold keys and have a ton of 4 and 5* cars maxed out. So it's pretty safe it seems as long as you don't go too crazy. I'll update again in a few days.