I have always tried to keep the stock sized circumference to keep the speedo correct.
So lowering your car makes a difference but altering rim height does not because larger rim equals lower profile tires.
There are only 3 reasons to alter the rim size.
#1 - larger brake rotors/calipers
#2 - Optimize handling characteristics
#3 - looks
It is my opinion that altering the total diameter of the tire is dangerous. It can alter the geometry of the steering and wheel travel and not properly done, can cause accidents.
With all the tire sizes available, you should be able to get a tire to fit on any rim less than 19" and keep the overall same tire height (within a few millimeters anyway).