even a quick email to the grad secretary or dgs is enough, if they need more they will ask. the biggest thing is to decline those offers you know you do not want as soon as you are sure so that these schools can make offers to others. there is no reason to string folks along and most will really appreciate a quick "no thanks" as opposed to a long letter explaining your reasons on April 14th. if you are holding on to multiple offers you are essentially keeping others from getting on and getting funding.
