To smile or not to smile….
Singing is usually a sublimated emotion weaved into a story FOR the audience– singing is not about self-created tones.
Smiling certainly has a number of advantages– it usually engages the audience, it probably does a number of things to relax the vocal apparatus, etc.–but smiling definitely doesn’t help a tragic or sorrowful tale. Most rock song have happy melody and sad lyrics and many if not most country songs are sad, so smiling is appropriate when both melody and lyrics are happy; maybe when one is thus, and inappropriate when neither is.
For rock, sometimes you should growl, for stage, sometimes you should frown…. you get the idea.
Expression that matches the desired emotional effect for the audience– not only will this help the audience, but because the body emotional expression matches the song emotional expression matches the audience perception of such emotion, and most importantly, match the intricate muscles nature designed to make such emotion. Singing should match the emotion as part of the story told– smiling is just one of these emotions.