Hello,
I've been working as a boom operator for a number of years and have starting working regularly as a mixer over the past two years. My strategy as a boom operator was always to put the actor's voice into the "sweet spot" of the mic as best I could. If there were a number of actors this might require very stealthy cuing, a wider pattern mic, or two boom operators. My idea was to be a close as possible to the edge of the frame line.
As a mixer, I'm listening more for evenness in the overall scene, balance between the voices of the actors and some generally consistent sense of place as far as the sound goes. It seems preferable to having the sound of a voice come in and out of "focus" as the boom operator works the scene.
I wonder how other mixers and boom operators approach it.