If you have been accepted somewhere you really like, then by all means withdraw from anywhere you would not want to go.
I withdrew from all but 3 of my schools after I received an acceptance from a school where I really love. Some schools took my withdraw over the phone along with my interview cancallation. Some I emailed and one I fax.
The reason you should do it, is because it gets you out of their system and frees up the space for someone else to be considered for an interview or acceptance.