Best Practice with Apple Business Manager

Typically, any devices owned by the organization would (should) be enrolled in ABM, as that gives you the greatest degree of control.  There's nothing wrong with only using Open(manual) enrollment, but you lose the ability to make the MDM profile mandatory (ie, not removable by a local admin).

https://docs.jamf.com/10.31.0/jamf-pro/administrator-guide/Computer_Enrollment_Methods.html

Just using the open enrollment option for your devices doesn't fully manage them. Ideally, you'd use Apple Configurator and build a supervision within that, which can point to your ABM I believe.  Also, Open Enrollment doesn't grant you full supervision - only done on first launch of device, through ABM, or a wipe, etc through Apple Configurator.  Are these company purchased devices? Can you justify to your users the need to wipe/re-enroll them? Might be more headache than gain. 

Yes, all devices of which I speak are company purchased.  But since they were purchased directly on the Apple Store or from an Apple Sales Representative and not through a private Apple Store set up for our organization, they don't appear in the ABM. I'll have to figure out how to best deal with these devices.  Some may be too old to even get them into the system as they lack T2.