This is not from personal experience, but don't some tournaments allow you to change lists between battles? If that were the case, I would say pick at the beginning of the battle. Otherwise, stick to one. If everyone has the same list, it is a cheese way to try to get the benefit of two lists.Since the codex doesn't specify the timing of choosing which to use I would say it would be up to the TO.
Personally I would say you must stick with one for the duration of the tourney.