What would be the consensus for the proper way to terminate roughed in NM cable IF it is already terminated in the panel? Taped end of cable? Twist-on connectors on individual conductors? Box required?
I'm thinking here of work that will require completion after finish carpentry or cabinetry so we have free ends of cables with slack to reach final locations.

It should never be landed in the panel util the load end is complete.
Don

Do you mean at the other end of the wire opposite the panel board? If so I would think that you have to "Safe off" the wire in a box and cover. This way if someone does energize it by mistake, the wire is safe inside of the box. The wire in the panel board should be labeled and safed off. The label should let you know where the other end of it is. For example, To box in attic for future use. Or To box in basement, garage, etc. for future use.

Here in conduit land we'd simply term the wire in a 1900, anticipating using a length of flex for the finished install (dishwasher / dispoaal, etc).

Some of our guys would "nut 'n' tape", other would simply bend the wire back over itself and tape the end.

It was also very rare that we'd connect to a breaker before the "destination" was connected, whether it was a receptacle or an appliance. If we did, we'd tape or LOTO the breaker in the "OFF" position.