You should take regular back-ups of both your site files and database, especially after major changes or if you are upgrading your theme version or the wordpress software itself. Occasionally plug-ins will stop working after a software update and if it happens to be a critical one you’ll probably want to revert back to the old version. Theme upgrades usually won’t break plug-ins but can cause display issues so having a copy of the working version available will save the day. Using child themes when you build the site can alleviate many problems that occur when making a theme upgrade. A child theme is a separate Cascading Style Sheet that overrides the theme style sheet. Theme style sheets are reset to the default upon an upgrade so if you made changes to it they will be lost. Your child theme will not be affected. We use child themes and/or custom style includes on all of our wordpress sites for this very reason.