The best time to perform the DB backup is before you install the hotfixes. The reason for this is so that you can restore the DB to a time before the hotfixes were installed in case something unexpected happens, such as your application no longer starting (a worst-case scenario). In other situations where either the hotfix isn't working the way you'd expect or causing some other unwanted behavior, you can simply roll it back (if your version of Pega has that feature). If you want to go to the additional effort of backing up before committing the hotfixes, you can do that if you choose, but the Pega recommendation of taking a backup is to allow you to go back to before the hotfix(es) were installed.