I always use the strap. It is there for a reaon. It isn't a spare part manufacterers add just so they can raise the price a few cents all while helping out their friends in the velcro industry.
I have tried not using the strap, or just opening the velcro to max size, and it causes far more arm/shoulder/neck straing than not using it. And, for those using detectors with plastic arm cuffs, not using the velcro strap, you are putting a lot of stress on the cuff wings every time you swing your detector. The faster you swing, the chances the cuff wing breaks goes up.