There are two things:
1. Should I upgrade to 3.9?
2. Do I need to run cron.php regularly?
Q1. It depends. If your Moodle release is still supported https://docs.moodle.org/dev/Releases and you are happy, then you can wait. If you need to upgrade, pl. ask in the Installing and Upgrading forum.
Q2. As highlighted on https://docs.moodle.org/en/Cron, "IMPORTANT: Do not skip setting up the cron process on your server for your Moodle. Your site will not work properly without it."