Everybody will tell you that you need to look approachable, you need to be smiling in your headshot.
If your smile is not authentic it will look false and it will be a turn off. People will not trust it.
It’s better to be seriously authentic than smiling phony. The important thing is to not look angry. In fact, a serious look can be attractive.
For business-to-business professionals, a more serious expression can be very appropriate.
A soft smile can be done very easily by anyone and it will relax the mood of the headshot.
You don’t have to always show the pearly whites.