When I started my old pony mare, she had the transitions, leads, most leg cues, and most vocal commands down. I only do small local gymkhana's, so no big deal there. And guess what? After gymkhana-ing most of the summer, she was still completely good with it, and never freaked in the arena like many barrel horses I've seen. She was a real nice all-rounder.
Could trail ride great, gymkhana great, and just "fun ride" great.
Now if it were going to be a pro racer, i'd say finish them off before starting barrels, because they will be competing long and big-time, usually.
I', a firm believer in a calm horse when they enter the arena, and a calm horse when you exit the arena, while listening in between going in and out.