Do I need to stop nextcloud will backing up with borg?

If I use borg to backup Nextcloud folder and storage, do I need to stop nextcloud?

Are there reasons it would be better to stop it?

If you do not stop it, the backup might catch some weird data for files which are saved while they are backed-up. Also, you should backup the database, same issue. If the database snapshot and the data backup do not match, you would run into problems when trying to restore the installation.