When I attempted my first cleanse a year ago, I did not quit coffee prior to starting my cleanse. What a miserable mistake. Just hideous. So bad, in fact, that I had to stop the cleanse at day 2.
Last March, I weaned myself off of coffee before starting the cleanse and had no problems. What worked for me to wean was to start decreasing regular coffee (when brewing at home) by adding a spoon of decaf and subtracting a spoon of regular every day until I was 100% decaf.
I can't imagine adding the horrors of caffeine withdrawal to The Master Cleanse mix. It just made me cave in.