When creating portraits of people, I want to capture their personality. I want my subjects to be relaxed, even as they look into the camera. It helps tremendously to develop a rapport prior to a sitting. Also, to converse with them during the shoot. Asking them to smile does not yield the same results as catching them in a natural smile.
Whether it's a headshot, a formal portrait, or a family, my approach is the same. Natural, relaxed, and comfortable looking portraits are my goal.
It is an unnatural thing to stand in front of a camera. The more relaxed your subject is, the better your images will be. Trust me on this. Do not force a smile. Do not ask anyone to say CHEESE. Of course you may get them to smile. Tell a joke, play with a child, use a squeaky toy with a baby, etc. Anything that will evoke an emotion that creates a genuine smile is great! But for me, a smile is not the goal. A pleasant, relaxed look that reveals my subject's personality and mood is.
It is not whether they smile or not that will make the portrait better. It's what you see in the eyes. Are they enjoying the experience? Are they relaxed, comfortable? These are the tangibles I look for. If not, then I am not doing my job.
Remember, when opportunities arise, take your best shot!