Rear Dewclaws

My bichons don't but they have popped up before, and they're usually attached and dangling by a small ligament and generally removed at a few days old. 
Not sure about PWDs since I'veonly had Echo, but she does not have rears, only fronts.  I am very glad her breeder left her natural, she uses her dews all the time to hold toys and chewies.

Not really addressing the breed deal, since he's a mutt, but Harry has his.  They're detached, he doesn't use them so it's difficult to keep the nails a decent length, but they're just kinda "there."  There is a chance he could rip one on something one day, but so far he hasn't.  Unless he does rip one and requires surgery, in which case we'd just have them both removed, he'll keep them.  It's kinda neat, I guess...not something you see every day.  Just one more thing that makes him "special." LOL.

Tootsie had fronts, but they were removed prior to me.

Fluffy, the Cairn, has rear dewclaws. I will probably have them removed, the first time I have a dental done on them. Somebody will probably throw me under the bus, for that, but they're a PITA to keep trimmed, and I don't like them.

I don't know about Bean. All of hers are removed. They do pop up in Cresteds, though, on occasion. I groomed a Yorkie, the other day, with DOUBLE rear dewclaws, like a Pyr. That was awesome. NOT!

  I had them in one out of my four litters - in that one litter at least half had one or more rear dews. I believe the sire of the litter had them - he's my dog and I can see a bit of a scar on both hind legs where they would have been (breeder didn't remember by time I thought to ask). I've seen or heard of them on other Belgians and have seen them on lots of other breeds at the grooming shop. Beagles are a breed I've seen them on pretty often but most of the Beagles I see are poorly bred.