there are a lot of people with IQs who simply don't join. it costs money, and there is a perception of it as being self-serving and smug, existing solely to gratify the egos of high-IQ people who are insecure about themselves. at least, this is what i was told by a friend who used to be a member, though of course i don't know how true it is or not.
if it was on a personal statement i'm sure it wouldn't hurt, but remember that the PS is supposed to advertise your curiosity, aptitude, potential, etc, rather than a raw statistic. remember as well that IQ points are solely a measure of your ability to analyse statistics/think scientifically, so may not be the most relevant if applying for an arts subject.
still, no harm in putting it down. it's just something that people tend not to shout about, i guess.