A check valve isn't completely necessary. I have 4 overflows running and none of the pump lines have check valves. All you need is a siphon break (hole in the pipe) right at or just below the waterline. That way, when the water level drops 1/8" or so, the hole is exposed to air and breaks the siphon, limiting the backflow through the pump.
IMO, if you are relying on a check valve, its an accident waiting to happen. If you get a snail in there, or even some algae buildup and the valve is unable to close completely, then if you lose power, you could potentially overflow the sump if you aren't there to catch it. I'd rather rely on a more robust failsafe.