By smiling you are showing that you are confident about yourself and so you make yourself more attractive to women, so yes it is important to smile.
I know how you feel because few years ago I had my tooth knocked out in rugby accident, they were put back in but one of them which is the front upper jaw tooth has internal bleeding so its slightly bliue.
So then I smile, its not a stupid massive one

but more of the closed lip smile, however im geting it treated however it takes long time but unlike you it can be done quickly, stop smoking, get whitening toothpaste, get good elecktronic toohbrush, clean your teeth at least twice a day for at least two minutes, floss and use mouthwash, if you got spare moneu then get them whitened by dentist.
Also dont lose your confidence over the slightly yellow teeth because no matter how white your teeth are and how looking you are women want a confident man.