I drafted a single letter stating that I was withdrawing from the school but that I was grateful that I had been accepted and wished them luck with their current applicant pool. I signed the letters and snail mailed them. I figured, you never know what can happen and if I ever wanted to attend their school for any reason, they could look back and see that I had been courteous and professional in all my previous correspondence with them. Also, they accepted me... so why would I be rude to them?
It took 20 minutes total of my time to withdraw from all the schools... in the grand scheme of this whole admissions process that is nothing at all.