Bill,
I made some both ways, and both are easily tight enough for the purpose.
My thinking is this that it only really matters if the sides aren't 
parallel, like a coffin smoother.
It is a little bit of a stretch to get the sides with tails to come in 
from the sides to go over pins on the base.

http://www.petermcbride.com/metal_plane_making/images/image05.jpg

compared to tails on the base coming up from under the pins on the 
sides, where the curve is irrelevant.

http://www.petermcbride.com/metal_plane_making/images/image16.jpg

These pictures are from presentation to our tool club, followed by an 
article I wrote for our tool club magazine in 2008.

http://www.htpaa.org.au/images/media/Tool_Chest_89_Cover.jpg

A link to the full article is here...

http://www.petermcbride.com/metal_plane_making/

Cheers,
Peter