I would always recommend flushing a steam generator as part of proper equipment maintenance.
I highly recommend purchasing the automatic drain valve on the steam system you select as this allows for a "hands-off" approach to maintenance and is just a good idea.
If you have an appliance/equipment maintenance routine for your home, just add this task to the list for when you, say, flush your hot water heater which processes far more water than a steam generator. Most steam generators are equipped with a drain line and, when installed with a ball valve, allow for simple manual draining of these units.
How often you should flush is different in every installation and takes us right back to which type of homeowner maintenance do you prefer - hands-on or hands-off? If prefered, you can monitor the water that gets manually drained from the unit and adjust your maintenance frequency accordingly. As I said, I highly recommend the automatic drain valve.
Whatever your preference, be sure to check out the product manual and warranty to verify the manufacturer's maintenance requirements.