Is it safe to update PFSENSE if you bought an SG Series from the store? Basically should I worry about having a firmware issue or are the store units tested for updates? Is the update process the same for the purchased units?
The considerations are no different than with other hardware in general, except you don't have to worry about any kind of potential hardware compatibility issues, which takes away the biggest risk there. As always, pay attention to the release notes posted on the blog. Generally fine to upgrade to latest at this point with the exception of those on 2.2.x or earlier reliant on a package that's been deprecated in 2.3.x+.
The upgrade process is the same.