I have no idea what the common practice is, I do know what the engineers planed on and what we tried to teach our clients.
Preamp volume controls are not perfect, the taper for each channel will often be different.
The designers try to have a range, say from 9:00 to 1:00 where the two channels track closely. The designers expect therefore the owner will set the amp gain controls to keep the preamp volume control to stay as much as possible in that zone.