Don’t Forget to Backup Your WordPress Database!
Either manually or using one of the numerous free WordPress backup plugins.
Backing up your WordPress site Is VERY IMPORTANT. The WordPress database contains all your posts, comments and links. If the database gets accidently erased or becomes corrupt you will lose everything. Remember to backup bi-weekly, weekly, or daily, depending on how active your WordPress website is. You could even backup after you’ve finished posting a blog or replying to comments.
WordPress section on Backups http://codex.wordpress.org/WordPress_Backups, however the simplest way is to automate the whole procedure, see WordPress Automated Solutions http://codex.wordpress.org/WordPress_Backups#Automatic_Backups
It doesn’t matter now or when you backup, just as long as you do backup.