It's optional, but as its allows the CPU to use only the speed it needs for running the application being used, it's common sense to install it, It allowed my CPU (Athlon 3400+) to run 6 Deg Cent. cooler. You can down load the AMD PowerNow Dashboard Demo to see the heat and energy savings being made.
I have had my AMD Athlon 64 3200+ PC for over a year now. I would have assumed (rightly or wrongly) that the 64 bit driver would have been installed by default at the factory.
As things go I am getting excellent speeds from the chip with multiple graphics programs open and multi-tasking on the net at the same time. CPU usage is rarely at max. So from that I would imagine that the driver was pre-installed as would be on most pc's that came with the 64 bit chip.