I generally meter the light that is illuminating the part of the subject that I want to expose best. This does not mean the strongest light in the area in all cases (the Sun when outside). I meter the sun only if the subject is being lit directly by the Sun. In shade, the Sun is the original source of all the light, but the light on the subject is not directly from the Sun. It is from reflected Sun light. So, I meter the reflected light.
If I want a bit more on the neg on the darker side, so that I can dodge it back, I will average the reading between the lit side and the dark side by placing the dome halfway between the two sides, i.e. at 90 degrees from the main light. In this case, a compromise is made to be able to dodge; the brighter side of the subject is overexposed in order to allow more to be dug from the darker side in printing. If I want a proper exposure for the dark side, so that I can dodge till the cows come home, I will take a reading from the darker side; I do this knowing that I am gaining the ability to print extreme shadow detail at the expense of grain and tonality changes on the lit side, due to the overexposure. I do this rarely, and when I do, I usually underdevelop the film quite a bit.
Try doing anything but method 1 on a digital camera or slide film, and you overexpose the lit side your subject; the greater the illumination ratio, the greater the overexposure when you meter down the middle or meter the dark side. You can pull E-6 film enough to help if you meter down the middle, and I suppose if shooting digital you can try highlight recovery, though it will look weird in anything but minor overexposure of the subject. But if using these media, exposing for the subject and filling the shadows is the best way to go if you want to have a tighter illumination ratio than what nature is giving you.
In flat lighting, such as an overcast day, or the aforementioned shady scenario, it doesn't matter all that much where you point the meter – left, right, or down the middle. If the subject is facing the camera dead on, and light is coming from all directions pretty-much equally, then the dome can follow the nose and it will work fine. But it doesn't follow the nose as a rule. If, after the dead on shot, the subject were to turn his or her head 45 degrees to his/her right, I would then meter the light falling on the left cheek. That means that the dome faces away from the cheek as if it was a huge boil on the subject's face: 45 degrees from the centerline, the other direction from how the subject turned, i.e 90 degrees from the nose.
Of course, if the light is the same where you are as where your subject is (which it often is outdoors, since the Sun is so far away), you don't even need to walk up to the subject. I only bother putting the meter to the face if the light is different where I am vs. where the subject is.
In any case, I never consider where the camera is. It is not relevant, beyond the obvious fact that I meter to get a good exposure for that which is shown in the frame (i.e. not the back of the subject, unless I want a silhouette). Hand held incident meters are for measuring incident light independent of the camera, not for making a composition-based reading; you use hand held meters specifically to avoid having your metering tied to composition. But by pointing the dome at the camera all the time, tying them together is exactly what you'd be doing. You'd be ignoring the lighting entirely, and chaining yourself to metering based on the angle of the film plane; you'd be missing out on a lot of the information that the meters can give you; and you'd be wasting a lot of the control that you can get with incident meters.
If you shoot negative film and meter toward the camera all the time, you will survive, and you may never notice unless you skillfully pay attention to exactly how your negatives look and how far away from "normal" your prints deviate. But you are shooting yourself in the foot if you do this when shooting transparencies or digital.
Studio versus natural makes no difference in this. It only makes a difference in the amount of control and time that you have. It is easier to craft the exact lighting ratio that you want when in studio.