In my experience yes. This will depend on the type of guy you are trying to attract but in general its best to smile.
Happiness is something everyone wants. And one of the things that guys really want is to feel good about themselves (they’re so sensitive). So if you smile when you are around a guy that you want to notice you then he will remember that he felt good around you and associate that good feeling with you.
Lots of people talk about chemistry. I think most girls believe that chemistry something mystical and devine like true love or love at first site. But I believe that chemistry is mostly your current mood which is often shown by your facial expression.
So. you should smile.