DD's coach bases it on which direction they naturally twist better. His opinion is that the round off involves twisting, and future skills that involve twisting should continue twisting in the same direction as the round off, rather than forcing the gymnast to twist in a direction opposite to their round off. So, he has them stand, jump straight up, and twist - several times in both directions. If you watch a kid do this, it doesn't take an expert to see which direction they're better at. He bases the "correct" leg on that, saying it will benefit them when they get to higher levels. It was a big enough deal to him to put DD through 6 months of endless round offs to get comfortable with the leg he wanted her to use.