Keeping a backup of your website is a wonderful idea. It provides insurance against all types of setbacks, such as security compromises and inadvertently deleting your entire site (it happens).
We suggest backing up your site before you make changes each and every time. That way, if something goes awry, you have a fresh copy that you know is working. It might seem like a hassle, but the first time you use a backup, you're going to pat yourself on the back for having so much foresight.