It's a 10 minute conversation, the first two times it was worth 150k and the last time was worth 500k. (Not counting the intangibles and immense benefit to later stages of the business)
Not practicing would have been a mistake.
Anticipating the types of questions we were going to be asked and prepping in a series of mock interviews with a few different YC founders who have also experienced the interview help to make the answers reflexive and such that we could steer the conversation to our strengths.
We didn't get in, but it wasn't our ability to talk about the business that did us in, based on our feedback.
Practicing for high stakes one offs helps you keep things tight and see weak spots before actually having to do the thing.