I can remember years back there was a FAQ regarding sexing by seed with photo of seed that wasn't even cannabis. Many people tried it, and confirmed it's a myth.
Not great experiences for me with fem seeds, so don't use em. I do however sex all my seedlings before week 4, using an eyelupe to differentiate preflowers. Current grow at day 21 from seed germ with 15 seedlings under a 400mh and just identified 3 males that got booted. I count on early sex detection to allow for prime lighting for the females. Don't won't to increase light/footprint and waste on males.