Depends on the manufacturer, but I'd still say it's worth it on the off chance that there's an issue with the cell. A check valve is cheap insurance for a big investment on a heater.
VT,
I think you missed the whole point. A check valve is required when you use a tab feeder because when pump is off the highly acidic tab feeder water can flow backward into the heater. A SWCG does not have anything that can flow back into the heater. When it is off, it is off there is nothing there. Adding a check valve is just not needed. I agree that it won't hurt anything, but why add anything this is not needed?
Thanks,
Jim R.