Jingle singing requires surgically accurate precision in everything... pitch, tone, phrasing, rhythm, entrances and cutoffs. Hired on because I could do most of these things well, I had a little trouble hitting a note just a bit low to the center of pitch one day. He told me to smile as I sang it... it worked like a charm and I never forgot it.
HOWEVER... there is a right and wrong way to apply this 'smile-to-raise-pitch' technique. It needs to be an 'inner'... NOT an 'outer' smile.
The inner smile looks like the Mona Lisa smile. Imagine you see someone coming into a room, you're very happy about it but don't want them to know. You will usually assume an inner smile. It lifts the soft palate in that beginning-yawn stretch, activates the eyes which lifts and activates the upper pharanx (back of nose) and not only is your pitch better, your tone is richer.
On the other hand, the outer smile looks like a grimace, or like the sun is in your eyes. This lifts the outer eye/upper cheek area in a way that actually tightens the jaw, flattens the palate and thins the vocal tone. Getting rid of this pervasive grimace has set many of my vocal students' voices free of strain and other limitation. Yes, for a moment in a song, a grimace can give a certain edge. I even use it myself from time to time. But it is not a cure for flat pitch.
One last point is to remember not to sing with a 'smiley' tone if it is not a happy song. To smile on songs like "I'm Yours", Happiest Girl, Amazing Grace, "You Make Me So Very Happy" sounds right. Smiling on songs like "Rollin In The Deep", "King of Anything", "Heard It Thru The Grapevine", "Chiseled In Stone", "Poison and Wine" sounds fake. You CAN use the inner smile, which does not have to have this inauthentic smiley characteristic.
So smile... wisely!