If a serious pose is your best look then by all means, show yourself with a serious look, but if you're best photographed with a smile, then I say smile wide for the camera. Only you know your best look and what might sell you to a perspective mate.
This is also true of the lady that I am about to show you. Although this 58 year old lady did show a rather average frontal pose, it was the following pose that made her most notable. Have a laugh.