Here's my 2 cents worth:

I have a Guild D-4 with a sculpted "fiddle-type" back that is one piece. Other guitars I've had have been made with two-piece backs.

Among those guitars I've owned, those that sounded good did so regardless of how their backs were constructed.

I'm sure there you'll get plenty of informed feedback from many of our more luthier-savvy Mudcats.