Agreed. Assuming the underbite is minor and does not cause issues with eating etc, I see no problem with getting a companion pet with this issue. One of our PWDs had a perfect bite as a pup that went undershot asI'm honestly not sure why so many people make such a big deal about the bites. There are several threads concerning it on this forum. Unless you're looking for a show prospect and/or a dog you may breed I have no idea why it matters so much for companions. Unless it is very very bad it causes NO issues with the puppies. Having said that the only issue I CAN see is if you're looking at a breeder who produces puppies like this often. Obviously there is something wrong with this breeder's understanding of the breed standard and one wonder's why they are breeding and what their goals are with their breeding programs. I breed and show dogs, and of course bites are important but if we get the occasional over shot mouth it's a bummer but we don't freak out about it and we know it isn't going to affect this puppy's life as a great pet.
My son has an underbite so I'll tell you what our human dentist has told me about the long term issues.
Because the teeth do not fit together correctly they will rub in spots not intended to take pressure and they will wear down over time. It can also lead to tooth loss.
I think bites are a big deal especially an under bite. I would not buy a companion puppy with an underbite because for me it is a physical issue and not just a cosmetic one.
I think it would depend on how undershot the bite is. For me I'd just not like it as a cosmetic issue but a lot of people wouldn't care about something like that. Harry's bite was off with his canine teeth when he was a puppy. The fixed themselves when he lost puppy teeth.