It's personal preference. It makes sense to picture frame if you can't undercut a hearth. Otherwise you end up with trying to butt end cuts against the hearth that look neat.....kinda what board ends look like that don't go under a baseboard or shoe molding. With picture framing things usually work out best if you start with the picture frame and install from there.
To see if you like the no picture frame layout lay some boards around the hearth and see if you like the look. If not, mark out the picture frame layout around the hearth and see if the cuts work.
I like no picture frame and having undercut the hearth you've got the pia part of the job done.