I do update/upgrade(usually dist-upgrade) as and when I feel like it. I should do it more often, but there are two things standing in the way of doing it very often. The first is that I would have to refresh at least 10 Pis, and that takes a while. The second is that some of them have various tweaks--different wallpaper, no wallpaper, color setting changes, modified default terminal geometry--that mean that I have to get out my notes and reset things that get overridden by the upgrade. So doing the job right is not only time consuming, but a right pain as well.
If those points aren't issues for you, then a weekly refresh is probably a good idea.