With over 200.000 cash per day its a bit difficult NOT to be a millionaire. Note that I have only 20 white houses in my town and only 12 of them are carefully hoarded somewhere more for fun than need of quick cash.
Hoarding helps when you have not reached the maximum level. Of course sensible players do it either as a real town planning or try to have it as far as possible from their carefully designed town.
After a while you dont need to hoard houses but a few of them dont hurt.
The real boost comes from:
1) Multipliers!
2) Premium Characters!