Well... two of my dogs... my lhasa/bichon and my yorkie poo have underbites. Bentley's isn't noticeable unless you physically open his mouth then you can see the bottom teeth stick out further.. he looks like he's smiling though and I didn't notice it much (he's 10 months). My lhasa/bichon has an underbite but still has her puppy teeth so I'm not sure how it will turn out.. the place I got her from told me she had the underbite, but she was too cute to leave based on that
*on a side note* my sister Lex who's 12 now.. she had an underbite as a baby and when she lost her baby teeth it actually corrected itself!! So you never know!