Personally, I check a few outside stats like the W3C and others, but I pay special attention to my own stats with regards to the deciding factor.
... But then again, we get about 500 000 visits a month, so we have some data.
Another thing to consider is the subject of the site: if you are building a site where your visitors are more likely to be up to date with newer computers, then IE6 becomes much less important. Sometimes though, it is hard to predict these things.
Stefan