There is No Correct Way to Smile
Women's magazines, since I can remember, have always been overflowing with tips on how to look good in photographs. Stick out your neck a little, press your tongue against your teeth, put your hand on your hip...endless ways to make yourself appear "perfect".
As a younger photographer I had shoots where I focused on these minute adjustments with the model, but ultimately when people are focusing on these strange ways to look "better" in photographs they end up looking awkward and it feels forced. What I learned was that engaging with the models in an authentic way, getting them to loosen up and genuinely smile, gave me much better results than fretting over the angle their head was at.
If it makes you more comfortable to focus on these tips when taking a photograph then go ahead and use them. But the issue is deeper than whether or not the tips work. Having these tips in every women's and teenage magazine affects our perception of ourselves. Tips how to photograph well ultimately just gives us a list of things we can criticize about ourselves. It is no longer enough to look at a photograph and see the joy you were having in that moment. All of a sudden your arm is position the wrong way and you start being self critical of your arms, when in reality you might have been wearing a tank top that you felt awesome in.
When I was growing up and hung out with groups of girlfriends I was always surprised by the number of different, odd things we were able to find wrong with our bodies. It has and still does take conscious effort on my part to not pick myself apart like that; I have to remind myself that nobody is going to notice my thigh dimple or the way my arm is positioned. The photos that I like the most of myself are ones where I wasn't thinking about being photographed but was just genuinely enjoying the moment. Be as kind to yourself as you are to the people you love.