For my laptop (Mac OS X): update as soon as updates are available, unless it's a major update (i.e. a new OS version, not just a patch). If it's major, I'll usually wait a bit and see how it goes in the wild with early adopters, first.
For my desktop (Arch Linux): Arch uses a rolling release model, so I do a daily check on what's available and then decide if I want to apply those updates immediately or wait. This depends on what is being updated. If it's the kernel or Xorg I might wait a bit. For minor stuff I usually go ahead and update right away.
For my phone (Android): At the moment I'm running Cyanogenmod. I don't flash new nightlies very often; I tend to stick with one I find stable. I do like to keep fairly current, though. I apply app updates as soon as they're available.
For my tablet (Android): It's a Nexus version and I never bothered rooting it or anything, I just get the updates as they come.