AKP just posted an article about Yoona (and Sunny) of SNSD giving A Pink (rookie KPOP group) 90 degree bows while on stage.  If you look at the photo they posted, it shows how Yoona and Sunny (more so Yoona) are at the back part of the stage, bowing to the rookie group.

It was far enough back stage where cameras usually do not reach, so Yoona wasn't doing it because she knew she would be photographed.

"That's just the way she rolls."

Our Yoona is so polite, full of respect and well mannered that ALL of her bows are 90 degree.  It's much like the way she introduces herself:  "I'm SNSD's Yoona Im."  I've never heard her say, "I'm Yoona Im."  No, she is so humble, so meek and unpretentious that she identifies herself as part of a whole, without trying to steal the spotlight for herself.

Just another reason why I love this girl.