Darkmatter,
I did something similar to that. Instead of email, I called them and told them that I thanked them for considering me, but that at this stage in the process I am leaning towards another school. I told them I would send a letter stating my decision as well, so that it would be filed.
I don't think withdrawing means anything. All that happens is some person in the admissions office has to go onto the computer and enter 'withdraw' or something like that.
As long as you didn't send a letter of intent to a school that you later withdrawed from...you are karmically cool. I think that is the only thing an applicant could do that would be looked upon as immature and canniving.