Being rich is only a problem if you rely upon governors to run and keep control of your cities. At 50K, 100K and 150K there are checks for about 6 or so bad vices. So if you have 150k+ then every governor is getting 3 chances to pick up one or more of those bad vices. Usually doesn't take long before the governors get bad enough that having them in the city isn't an improvement over not having them in the city. Sometimes they can actually be worse.

So if you don't generally rely upon governors, then the amount of money you have is irrelevant. Besides, if you have the amount of money needed to start getting the bad vices checks, then you can generally afford to move your governors out and simply use garrisons to keep the peace in all but the newly conquered territories.