last week the entire state of Kerala is under water.. the year before it was Chennai.. When it rains HARD, it doesnt matter how much storage we have, it will overflow.. and if you have a big roof you will need a LOT of VERY good drainage to deal with it.. if there is ever a time for overkill it is drainage as material cost is low, depending how you do it.. and as it does get a bit dirty or blocked int between cleaning it will still allow enough water to pass though..
If you are building with any wood near the ground that is just another reason for good drainage.. NOthin worse than rotting wood that needs replacing ;-)