I use the Prestone flush kit. It works fine. Tap water used while flushing the system won't hurt anything. I use Prestone long life coolant and mix with tap water to get the 50/50 blend. You can back flush the heater core as well by unhooking the hoses and putting a garden hose to the lower heater core nozzle. Sure, Prestone is not the Organic stuff specified in the owners manual but it does the trick and works just fine. I've never had trouble with it in my Jeep. The OEM stuff did corrode my thermostat, cause rust around the engine block where the thermostat resides and corroded the thermostat housing as well. That was my experience with it anyway.
Distilled water works fine, but, so does tap water. To each their own.