Preparation is a gamble - you're gambling that the small amount of extra (possibly unpaid) time you're putting in now will save you a large amount of (paid) time later. The odds are always heavily in your favor, since 99% of clients won't figure out all of what they want until the last second. Plus, having to rewrite your entire application every week will drive you insane rather quickly, and even if you aren't getting paid for anything except your hours, it's best to prepare purely for the sake of your mental health.