Honestly, I would never cut a clone off of a plant that hasn't been sexT unless it was the last of its kind and you wanted to possibly obtain pollen?
Seeds, in all reality it's most likely a myth about the shape determining the outcome of sex for the plant, Ive found that the bigger and dark watermelon stipe covered the better, and usually I get about a 70% female ratio just going for the dark ones. I don't think appearance or shape have much to do with sex, but can suggest other things such as how developed /healthy the plant that developed it. just my opinion.