If you have any sort of permanent connection you should update every day. Usually the mid morning to early afternoon signatures have been built to block whatever comes from overseas that day. But you have to do this since even what may seem as extreme precaution may not be enough, as the virus sometimes advance in front of the virus, although that actually did not seem to be the case with MyDoom, there are just a lot of people who aren't upgrading fast enough that got caught and accidentally clicked those files. Bottom line, treat the dangers of the internet with deserved respect, and you'll be fine.