I wondered about that myself.
On my photo, you can kinda see where the BB had split crookedly all down the wood grain (towards the left - it wasn't on a machined joint). I had to inject glue & clamp hard to stabilize it. It's solid now, but it left a visible void I preferred to fill. And fill again. And fill some more. I figure the original poplar glue-ups were nice and flat originally.
On the front-facing miters, you can see the seams on this Big Top IPDB photo, so you're probably right to leave those showing:
http://ipdb.org/showpic.pl?id=286&picno=28916&zoom=1
You can't see them in the photo, but my seams were coming apart pretty badly and couldn't just be nailed back down. The wood behind the front frame had swollen so bad I had to rip the casing strips off and plane the wood underneath back down. I used some dolphin glaze on that afterwards too - it's pretty hard to line it all up just right on the wood alone. Probably oversmoothed it, but this machine is more of a resurrection than a restoration.
Original BB here - it looks OK in the pic, but it's falling apart, more like the cab...
IMG_0629_(resized).JPG
IMG_0631_(resized).JPG