As photographers, we often want to create the most pleasing image possible for our subjects. We often go to great lengths to assure that the image is well exposed, adjust lighting angles, color correct, compose creatively etc.. Of course one recipe will not necessarily work for every subject in front of your lens.
Double chins can occur at any age, on males or females. That little extra mound of skin just below the chin can be very unflattering in some but not all cases. Remember, that we want to make our subjects recognizable as themselves and if our subject has always had a double chin, they may be fine being photographed with that feature. In fact, they may actually become annoyed if they receive and image and it appears as though they have had surgery to remove this tell tale feature. Communication is they key as always with your subject.
Lets assume for the moment that your subject would not be pleased with a photo displaying their double chin. One step in your suggested pose would be to have your subject lift their head slightly and extend their head slightly toward the camera. This tends to make the skin under the chin a little more taught. The movement however, should not be exaggerated or the subject will look like a turtle sticking its head out of a shell, again.. not flattering.
Extreme crops are of course an option, in which the image is cropped tightly so that the extra mount is not visible. Although this may work for one or two images, it may not be the best choice if a client wishes head and shoulders portraits. Lighting angles can help to a certain degree by placing the neck in relative shadow with very soft lighting.
Post production of course can help, reduce a double chin, with a variety of tools in photoshop including the patch tool and even the liquify tool.
Of course, whenever possible we want to get things right in camera. Creative poses such as shown below may help flatter your subject as well as save time in post production. As mentioned earlier, in most cases discuss your plans with your subject and even show them samples of poses that would best compliment your subject.
Above image set from Head and Shoulders portrait photography- JSmith.