I know for myself, I've been taking advantage of the ability to change our votes. I voted right away for the first one that grabbed me, and I've been going back in a more thoughtful way and adding in my second and third votes. If I thought they were needed. If I find that a second or third vote is only going to push a story by one more when it's already ten votes ahead, I'm holding back.
I don't know how ethical it is, but since we have the ability, I figured it could be considered ok - I'm not removing any votes I cast, just adding a few in when a story I liked seems to be a little down. Also, I don't like seeing stories in the semifinals with only one vote.