We will have probably already provided some level of 'on spec' consultation/advice to close the deal and everyone is ready to move forward. Further work or beginning of project begins only after money is paid, which is usually straightaway. (People that write checks 'only on the 1st' or what-have-you usually find a way to write a check if it is the 6th and they want to get things moving:)) Clearing is rarely an issue, so we don't much worry about that. Only an issue if the near the end of the road something has soured, decisions reversed, project creep (but not wanting budget creep)... but this stuff usually smells from a long way off and we stay ahead on money, or at least even because of completed worked that hasn't been turned over or launched yet. Sometimes the last payment has to be cleared before we give away leverage.
Always to try to separate critical pieces so that there is always a key piece of worked absolutely needed in the next phase or payment block, or they are screwed. Makes it less likely that you'll be messed with so long as you control a key piece of the puzzle at all times.