I always keep the heavy one's and toss everything else back. If anything is lighter than the heaviest, there's a possibility it has an air pocket. I've had the best luck with the Lee moulds. Lyman steel moulds took too long to heat up and regulate, but they could produce quality, many times superior balls. Just too many rejects!
I have great results with my Lyman 69cal mold. Keep it hot and pour fast. I'm convinced that many folks who cast run the pot a bit on the cold side.