Unless it taxes system resources frequently, you can stick to to the auto defrag feature. As you use that computer more and more you'll be able to decide. Alternatively, you can turn off auto-defrag and schedule defrag or do a manual defrag once a fortnight or once a week depending on whether you download/install/uninstall a lot.
Personally, I have auto-defrag turned off (i'm a control freak
), and i have a routine consisting of AV plus temp files removal plus manual defrag once a week.