As everyone has said "you pick". In the case of two riders having almost identical runs an official will take into consideration a shorter vs longer rope length. If two people threw down runs that essentially scored the same, the rope length could be the deciding factor to place on rider higher than the other. That being said, ride with whatever you're comfortable with. It's about you doing your thing and having a good day!
In the odd contest they have familiarization which is a practice run but for 90% of the contests I've competed at, it's strap in and go. The biggest challenge is to be mentally prepared. You want to be relaxed and enjoying the environment. If you have addrenaline and nerves try to move them to focused instead of nervous.
Contests are really about having fun, meeting new people, and just trying to ride your ride.