What ever you do, don't use the check valves that you get at hardware stores. They have about 5x the head loss as a Jandy or Pentair check valve and will significantly reduce your flow. Spend the extra money for the Jandy or Pentair, it is well worth it.
I might get some grief for this but in my opinion, I believe that the recommendations for using check valves with SWGs is a carry over from the puck chlorine feeders. Pucks will continue to dissolve and the chlorine concentrations will rise much higher than what a SWG has left over after it is turned off. The chlorine rise of a typical SWG is only about 1-2 ppm and when shut off, they don't produce anymore chlorine unlike a puck feeder. So I think the recommendation is just to be conservative but if you really think about how a SWG works, I just don't see how the chlorine level could get that high with the pump off to damage the heater. Also, if you happen to have solar, the water will drain out of the heater towards the returns, so it isn't an issue in that case.
Some manufactures such as Goldline don't even recommend a check valve after the heater and add to that your SWG is 15' away from the heater, I would say don't bother. But if you want to be safe, go ahead and put one in, just make sure it is a flapper type of check valve. Also, if your heater manufacture requires it for warranty, then definitely put one in.