We don't do a lot of new resi construction, but from what Ive seen (existing), most bathrooms are on the same circuit. On that note, if you have no power on the downstairs bath, check to see if the upstairs bathroom GFCI outlet has tripped. :roll:
On the few we have done, I think it more a convenience to run separate circuits to 2nd floor bathroom(s), especially if the upstairs bath is closer to the panel. I prefer to wire it like commercial (hotels): assume it's going to be (ab)used. That means mom+2 daughters* will be running 3 2kw hairdryers simultaneously, as well as having every light on possible. Probably overkill, but I hate rework, especially when I just installed it. Crawlspaces and attics kinda stink no matter what....
*Nothing sexist here, just an observation.