Well it's always good so save up a good amount since the interest per day is 10%.

When you've got the 5000 for the first upgrade if you keep those 5000 for just one more day you'll get 500 (which is more than the sum of 2 normal salaries).

I didn't upgrade anything during the 1st generation. The reason for this is that by the time they had the money, they had already naturally made it to level 6-7. No point. What if the 2nd gen uses a different area? So, I upgraded the area as soon as I got my 2nd gen person and knew what area would be used.

I don't spend money if I can help it. The interest is what makes it add up fast!